NRLF 


B    M    31M    M7fl 


r- 


THE 


PRINCIPLES  OF   BOTANY, 


AS  EXEMPLIFIED  IN  THE 


PHAIEROGAMIA. 


HARLAND     COULTAS 

Professor  of  General  and,  Medical  Botany  in  the  Penn  Medical  University  of 
Philadelphia. 


PHILADELPHIA: 
KING    &    BAIRD,    PRINTERS,    No.  9    SANSOM    STREET. 

1854. 


BIOLOGY 
UBRARy 

8 


Entered  according  to  the  Act  of  Congress,  in  the  year  1854,  by 
HARLAND    COULTAS, 

In  the  Office  of  the  Clerk  of  the  District  Court  of  the  United  States,  in  and  for  the 
Eastern  District  of  Pennsylvania. 


DEDICATED 


WILLIAM     SCHMOELE. 

. 

DOCTOR  OF  PHILOSOPHY  AND  MEDICINE, 

PROFESSOR  OF    GENERAL  AND  SPECIAL  PATHOLOGY  IN  THE 
PENN  MEDICAL  UNIVERSITY  OP  PHILADELPHIA. 


CONTENTS 


P«gc 
INTRODUCTORY  REMARKS.  Phanerogamous  and  Cryptogamous  plants. 

Analogies  between  the  organization  of  plants  and  that  of  animals.  9 


PART  III. 

ON  THE  ORGANS  OF  NUTRITION  IN  PHANEROGAMOUS  PLANTS 17 

CHAPTER  I. 

THE  EPIDERMIS  AND  ITS  APPENDAGES 18 

CHAPTER  II. 

TliE  DIFFERENT  KINDS  OF  STEM 25 

CHAPTER  III. 

ON  THE  ROOT  OR  SUBTERRANEAN  APPENDAGES  OF  THE  AXOPHYTE....   32 

CHAPTER  IV. 

THE  ORGANIZATION  OF  THE  STEM 46 

CHAPTER  V. 

ON  THE  DEVELOPMENT  OF  THE  BUDS  AND  BRANCHES 64 

CHAPTER  VI. 

• 

THE  LEAVE_S 72 

CHAPTER  VII. 

ON  THE  NATURE  AND  SOURCES  OF  THE  FOOD  ASSIMILATED  BY  PLANTS.       89 


CONTENTS. 


Page 

PART  IV. 

ON  THE  ORGANS  OF  REPRODUCTION  IN  PHANEROGAMOUS   PLANTS 107 

CHAPTER  VIII. 
GENERAL  CONSIDERATIONS  ON  THE  FLOWER 107 

CHAPTER  IX. 

THE   INFLORESCENCE 114 

CHAPTER   X. 

THE  FLORAL  ENVELOPES 125 

CHAPTER  XI. 

THE  ANDR(ECIUM,  OR  STAMINAL   ORGANS 138 

CHAPTER  XII. 

THE  GYMNCECIUM,  ORPISTILLINE  ORGANS 148 

CHAPTER  XIII. 
THE  PROCESS  OF  FERTILIZATION  OR  FECUNDATION 161 

CHAPTER  XIV. 

ON  THE  VARIOUS  MODIFICATIONS  OF  THE  REPRODUCTIVE  ORGANS 173 

CHAPTER  XV. 

THE  FRUIT,  OR  MATURE  OVARY 192 

CHAPTER  XVI. 

THE    STRUCTURE   OF   THE    SEED 208 

CHAPTER  XVII. 

ON   THE   DISPERSION   AND   GERMINATION   OF    SEED....  ..    220 


PEEFACE 


THIS  volume  renders  complete  the  previously  published 
work  of  the  author  entitled,  "  Principles  of  Botany  as  exempli- 
fied in  the  Cryptogamia."  In  that  work  it  was  shown  that 
the  same  laws  of  nutrition  and  reproduction  operate  in  the 
vital  economy  of  plants  composed  of  a  few  cells  as  when 
vegetation  is  constructed  on  a  scale  of  gigantic  magnitude  and 
grandeur.  In  this  new  volume  on  the  Phanerogamia  or 
flowering  plants,  the  author  enters  on  the  investigation  of  the 
anatomy  and  functions  of  a  more  highly  organized  and  complex 
vegetation.  He  has  examined  with  care,  the  writings  of 
Schleiden,  Lindley,  Balfour,  Gray,  Richard,  and  other  eminent 
botanists,  but  above  all  the  Volume  of  Creation,  of  which  every 
other  work  is  but  an  imperfect  copy.  The  book  is  illustrated 
with  numerous  engravings,  and  such  scientific  terms  as  it  was 
necessary  to  introduce  into  the  text  have  been  carefully 
explained  and  their  etymology  given  as  soon  as  introduced. 

The  author  takes  this  opportunity  of  gratefully  acknow- 
ledging the  kindness  and  liberality  of  the  physicians  of 
Philadelphia,  who  have  been  and  still  are  his  principal 
patrons. 


PREFACE. 

He  has  written  this  volume  on  the  organography  and 
physiology  of  the  Phanerogamia,  in  the  hope  that  it  will  be 
generally  useful,  but  with  an  especial  reference  to  the  wants 
of  medical  students  and  physicians :  life  probably  exists 
under  the  simplest  andleast  complicated  condition  in  plants. 

The  author  ventures  to  hope  that  this  volume,  (which  he 
has  been  encouraged  to  prepare,)  will  be  found  equally  as 
interesting  as  the  one  already  published,  and  contain  such  an 
amount  of  original  and  well-selected  matter  as  will  render  it 
worthy  of  that  liberal  patronage  which  has  been  bestowed  on 
his  previous  labors. 


INTRODUCTION. 


PLANTS  exercise,  in  common  with  animals,  the  two  princi- 
pal functions  of  organic  life — nutrition  and  reproduction.  All 
the  organs  of  the  most  complex,  as  well  as  of  the  most 
simple  plants,  are  developed  for  the  purpose  of  carrying  on 
one  or  the  other  of  these  two  functions. 

The  tissues  which  constitute  the  substance  of  both  animals 
and  plants  are  formed  from  cells,  and  exhibit  a  most  re- 
markable accordance  in  their  vital  phenomena.  In  both, 
peculiar  secretions  are  carried  on,  which  are  restricted  to  cer- 
tain parts  of  the  organism ;  whilst  as  life  advances  to  the  period 
of  its  close,  the  walls  of  the  fully  developed  cells  become 
thickened  by  the  internal  deposition  of  matter  in  layers. 
Ossification  in  animals  exactly  corresponds  to  lignification  in 
plants. 

Plants  as  well  as  animals  reproduce  themselves.  Flower- 
bearing  plants  when  they  arrive  at  an  adult  state  develope  male 
and  female  organs,  termed  stamens  and  pistils.  These  mu- 
tually operate  in  the  formation  of  an  embryo  or  seed,  which 
contains  within  its  folds,  in  a  rudimentary  condition,  all  the 
organs  of  the  fully  developed  plant.  These  embryos  are 
formed  in  a  particular  organ  termed  an  ovule,  and  are  de- 
veloped in  consequence  of  imbibing  the  fecundating  matter  of 
certain  cells  termed  pollen.  Thus,  from  the  vital  actions  of 

2 


X  INTRODUCTION. 

plants,  there  may  be  much  instruction  derived,  which  will  be 
found  a  valuable  contribution  to  our  knowledge  of  the  repro- 
ductive function  in  more  highly  organized  beings. 

In  the  animal  organism,  the  nutritive  and  reproductive  func- 
tions are  greatly  complicated  by  the  presence  of  a  nervous 
system.  In  plants,  these  two  grand  functions  of  organic  life 
are  carried  on  free  from  nervous  influences,  and  therefore 
under  greatly  simplified  conditions.  The  careful  study  of 
these  functions,  thus  simplified  in  plants,  ought  therefore  to 
precede  the  investigation  of  their  higher  and  more  complicated 
phenomena  as  manifested  in  animals. 

It  is  undeniable  that  the  plant  takes  precedence  of  the 
animal  in  nature,  being  elaborated  out  of  inorganic  matter  as 
material  for  the  subsistence  of  the  animal.  It  would  therefore 
seem  to  be  the  most  natural  and  philosophical  mode  of  investi- 
gating the  phenomena  of  life,  first  of  all,  to  see  to  what  extent 
its  functions  have  been  expressed  in  plants. 

All  organic  matter  appears  to  be  only  a  manifestation  of  life 
in  different  degrees  of  development,  and  a  plant  may  be  truly 
regarded  as  the  simplest  manifestation  of  its  functions. 

In  the  author's  "  Principles  of  Botany,  as  exemplified  in  the 
Cryptogarnia,"  it  was  shown  in  sections  50,  51,  that  the  sim- 
plest plant  in  nature  is  the  plant  cell,  which  "  constitutes  an 
entire  vegetable  without  organs,  imbibing  its  food  by  endosmo- 
sis  through  every  part  of  its  surface,  which  it  converts  into  the 
materials  of  its  own  enlargement  and  growth,  and  finally  into 
new  cells,  which  constitute  its  progeny."  But  as  we  advance 
in  the  scale  of  organization,  the  cells  thus  generated  do  not 
separate  from  the  parent  plant  cell;  on  the  contrary,  they 
remain  united  with  it,  to  a  greater  or  less  extent,  until  we  find 
individual  plants  composed  of  a  mass  of  such  cells,  all  mutually 


INTRODUCTION.  XI 

co-operating  in  carrying  on  the  nutritive  and  reproductive 
functions.  It  was  also  there  proved  (54-57),  that  "  it  is  not 
necessary  for  cell-development  to  be  carried  to  any  great  extent 
in  order  to  constitute  the  fabric  of  a  true  and  perfect  plant ;" 
on  the  contrary,  the  same  laws  of  nutrition  and  reproduction 
operate  in  the  vital  economy  of  plants  composed  of  a  few  cells 
as  when  vegetation  is  constructed  on  a  scale  of  gigantic  mag- 
nitude and  grandeur.  In  such  plants,  it  is  evident  that  we 
have  the  phenomena  of  life  existing  under  extremely  simplified 
conditions ;  and  if  ever  "  man,  the  minister  and  interpreter  of 
nature/'  is  destined  to  discover  those  morphological  laws 
which  govern  this  evolution  and  endless  repetition  of  the  same 
definite  forms  of  vegetable  and  animal  life  from  the  same 
embryos,  it  is  here  that  he  must  commence  his  investigations. 

Hitherto  the  attention  of  the  student  has  been  directed  to 
the  consideration  of  cryptogamous  vegetation,  we  are  now 
about  to  enter  on  the  examination  of  vegetable  life  as  unfolded 
in  the  more  complex  and  elaborate  organization  of  the  Phanero- 
gamia,  or  flowering  plants. 

In  the  lower  forms  of  the  Cryptogamia  the  essential  organs 
of  vegetation,  the  root,  stem  and  leaves,  are  blended  together 
into  a  flat  or  filamentous  expansion  of  vegetable  matter, 
termed  a  thallus,  from  whence  these  plants  have  received 
the  name  of  thallophytes  (OaM.b$  a  frond,  $vtbv  a  plant.) 
These  plants  have  no  vegetable  axis  or  stem,  and  increase  by 
additions  of  matter  to  their  periphery  or  circumference,  They 
have  a  tendency  to  grow  in  a  horizontal  rather  than  in  a  ver- 
tical plane,  their  spores  germinating  indifferently  in  all  direc- 
tions from  any  part  of  their  surface. 

The  cells  which  constitute  the  tissue  of  thallophytes,  in  the 
lower  forms  of  their  development,  appear  to  retain  the  form, 


xii  INTRODUCTION. 

and  to  exercise  the  functions  of  the  parent  plant  cell,  with 
which  they  remain  permanently  united.  Thus,  in  the  nume- 
rous tribes  of  marine  Cryptogamia  or  algae,  the  endochrome 
(sv8ov  within,  and  zpupa,  a  color)  is  diffused  through  the  entire 
substance  of  the  frond,  so  that  the  whole  plant  presents  the 
same  color  in  all  its  parts,  and  the  reproductive  matter,  or 
sporules,  makes  its  appearance  in  many  species,  ^indifferently 
on  any  or  every  part  of  the  plant. 

In  the  Phanerogamia,  or  flowering  plants,  on  the  other  hand, 
root,  stem  and  leaves  are  separate,  well-defined  organs. 
From  the  first  commencement  of  germination  there  is  a  stem 
more  or  less  manifest,  and  a  tendency  to  develope  in  two  oppo- 
site directions,  into  the  earth  and  atmosphere,  the  two  grand 
sources  from  whence  these  plants  obtain  the  materials  of  their 
growth  or  enlargement.  To  subserve  the  purposes  of  a  higher 
and  more  elaborate  nutrition,  certain  cells  of  the  parenchyma 
are  carried  to  a  much  higher  degree  of  development,  and 
assume  the  form  of  woody  fibre  and  spiral  vessels. 

The  pleurenchyma  of  flowering  plants  (26)  becomes  more 
distinctly  marked  in  their  leaves,  as  organization  advances  in 
complexity  of  structure,  until  at  length,  in  the  most  highly 
organized  plants,  its  fibres  form  a  beautiful  anastomosis  of 
veins,  veinlets  and  capillaries.  The  leaves  of  Thalictrum 
anernonoides,  the  rue-leaved  anemone,  an  early  and  exceed- 
ingly abundant  spring  flower,  furnish  an  admirable  illustration 
of  pleurenchyma  thus  ramified  and  attenuated. 

But  throughout  organic  nature,  a  change  in  the  form  of  any 
organ  is  always  associated  with  a  corresponding  change  in  its 
function.  The  secretion  of  the  cells  is  therefore  no  longer 
uniform,  but  varied  and  well  defined  in  its  character,  certain 


INTRODUCTION.  Xlll 

peculiar  secretions  being  restricted   or  confined  to  certain  por- 
tions of  the  organism. 

The  analogy  between  the  vegetable  and  animal  tissues  is 
beautifully  apparent  in  the  secretory  action  of  the  cells  of  pha- 
nerogamous plants.  The  same  endochrome,  or  coloring  matter, 
no  longer  gives  an  uniformity  of  hue  to  the  tissues,  but  the 
leaves  which  terminate  the  axis  of  growth  become  crowded 
together  into  a  beautiful  rosette  at  its  summit,  and  secrete  a 
variously  colored  endochrome,  which  has  received  the  name  of 
chromule  (^pw^ta  color),  in  contradistinction  to  chlorophyl 
j  green,  $Mkov  leaf),  which  is  the  substance  which  gives 
leaves  their  green  hues. 

But  the  possession  of  a  terminal  rosette  of  beautifully 
colored  leaves,  popularly  called  the  flower,  is  by  no  means  the 
principal  characteristic  of  Phanerogamous  vegetation,  since 
in  some  flowering  plants,  as  for  instance  in  the  grasses,  these 
colored  investments  become  abortive  and  rudimentary.  Yet 
the  organs  essential  to  the  formation  of  the  embryo  are  there, 
the  stamens  and  pistils,  and  it  is  the  presence  of  these  bodies 
which  constitute  the  true  flower. 

The  difference  between  phanerogamous  and  cryptogamous 
plants  consists  in  the  possession  by  the  former  of  stamens  and 
pistils,  or  true  flowers,  (of  which  the  latter  are  wholly  deprived,) 
by  the  mutual  action  of  which  an  embryo  or  seed  is  produced, 
which  is  a  much  more  highly  organized  body  than  the  spore. 
The  spore  from  which  every  cryptogam  is  developed  is  commonly 
a  simple  cell  filled  with  organic  matter,  and  the  organs  which 
it  developes  in  germination  form  themselves  as  they  appear ; 
but  in  the  embryo  or  seed,  these  organs  existed  before,  and  are 
only  increased  by  the  act 'of  germination.  The  character  of  an 
embryo  in  organic  beings  is,  that  it  contains,  in  a  rudimentary 

2* 


XIV  INTRODUCTION. 

state,  all  the  organs  of  which  the  organic  being  is  composed 
in  its  entire  developments.  Thus  the  animal  embryo  con- 
sists of  the  head,  the  trunk,  and  the  extremities, — in  other 
words,  of  all  the  parts  of  which  the  adult  animal  is  composed. 
In  like  manner,  the  embryo  of  a  phanerogamous  plant,  of  a 
bean  for  example,  discloses  a  plumule  or  young  stem,  a  pair 
of  leaves  or  cotyledons,  and  a  radicle  or  young  root, — in  other 
words,  the  entire  plant  in  a  rudimentary  condition ;  and  by  the 
act  of  germination,  analogous  in  its  effects  to  the  commence- 
ment of  life  in  infancy,  all  the  parts  of  the  plant  develope 
themselves  into  their  wonted  figure  and  hues  in  accordance 
with  those  generic  and  specific  laws  to  which  the  plant  is  sub- 
ject ;  but  germination  does  not  increase  the  number  of  these 
parts,  which  existed  before  its  influence  was  exercised  on  them. 


PART    III. 

ON    THE 

COMPOUND   ORGANS   OF    PLANTS. 

THE  PHANEROGAMIA,   OR    FLOWERING   PLANTS. 


PAET  III. 

ON  THE  ORGANS  OF  NUTRITION  IN  PHANEROGAMOUS  PLANTS. 


VEGETATION,  in  the  more  highly  organized  and  complex 
forms  which  it  assumes  in  flowering  plants,  consists  essentially 
of  a  continuous  axis  or  trunk,  which  developes  in  two  opposite 
directions,  and  is  more  or  less  ramified  at  its  two  extremities. 
The  superior  or  ascending  portion  of  this  vegetable  axis  is. 
called  the  stem,  the  inferior  or  descending  portion  the  root, 
and  the  point  of  departure  of  either  axis  the  collet  or  neck. 
This  neck  is  usually  distinctly  visible  when  the  embryo  plant 
first  rises  from  the  ground ;  after  the  cotyledons,  or  first  pair 
of  young  leaves,  have  developed,  it  disappears,  and  becomes  a 
merely  imaginary  line  of  separation  between  the  base  of  the 
stem  and  the  root. 

These  two  extremities  of  the  vegetable  axis  are  beautifully 
adapted  to  the  earth  and  atmosphere,  the  two  grand  sources  of 
all  vegetable  nutrition.  •  The  aerial  portion  of  the  plant  is 
provided  with  leaves,  by  which  food  is  taken  in  from  the 
atmosphere,  and  also  with  flowers,  which  are  the  organs  of 
reproduction ;  the  subterranean  portion  is  furnished  with  a 
quantity  of  fibres  or  smaller  roots,  which  make  their  appearance 
in  proportion  to  the  requirements  of  the  plant  and  the  barren 
or  fertile  nature  of  the  soil  in  which  it  grows.  This  vegetable 


18 


COMPOUND    ORGANS    OF   PLANTS. 


axis,  with  its  assemblage  of  nutritive  and  reproductive  organs 
at  its  two  extremities,  has  been  very  properly  termed  the 
axophyte. 

Before  commencing  our  exposition  of  the  anatomy  and  func- 
tions of  the  fundamental  organs  of  flowering  plants,  it  is  proper 
to  examine  that  peculiar  investment  which  covers  them,  termed 
the  epidermis. 


CHAPTER  I. 


ON  THE  EPIDERMIS  AND  ITS  APPENDAGES. 

EVERY  part  of  a  plant,  as  well  as  of  an  animal,  with  the 
exception  of  the  stigma  or  summit  of  the  pistil  and  the 
extremities  of  the  roots,  is  covered  by  a  thin  membranaceous 
investment,  termed  the  epidermis. 

Fig.  1. 


Pellicle  of  cabbage,  detached  by  maceration,  covering  the  hairs,  h,  and  having  open- 
ings, s,  corresponding  to  the  stomata. 

The  epidermis  consists  of  two  parts  :  1st,  an  outward  pel- 
licle, (Fig.  1,)  without  appreciable  organization  called  the 
cuticle ;  2d,  one  or  more  strata  of  flattened  tabular  cells,  which 
are  much  larger  than  the  cells  of  the  subjacent  tissue,  consti- 


THE   EPIDERMIS   AND   ITS   APPENDAGES.  19 

tuting  the  true  epidermis  or  skin.  These  two  superposed 
membranes  are  intimately  united  and  pierced  by  a  number  of 
apertures,  called  stomata  or  pores. 

The  presence  of  the  cuticle  on  the  exterior  surface  of  the 
epidermis,  may  be  detected  by  a  simple  chemical  process.  If  a 
transverse  section  of  the  epidermis  be  treated  with  a  dilute 
solution  of  iodine,  the  cells  of  the  epidermis  will  remain  colorless, 
whilst  the  cuticle  will  assume  a  yellowish  or  brownish  tinge. 

Some  writers  consider  the  cuticle  to  be  a  mere  secretion 
from  the  epidermic  cells  on  which  it  is  deposited ;  but  the 
recent  investigations  of  M.  Gareau,  a  distinguished  French 
physiologist,  who  succeeded  in  effecting  its  quantitative  analy- 
sis, would  seem  to  prove  that  it  is  a  distinct  organ,  formed 
from  cellulose  of  a  special  matter  distinct  from  that  which 
constitutes  the  epidermis. 

The  cuticle  is  the  only  part  of  the  epidermis  which  covers 
the  surface  of  the  stem  and  leaves  of  aquatic  plants.  It  is 
developed  in  the  form  of  a  glaucous  bloom  or  vegetable  varnish, 
which  renders  the  surface  of  the  plant  a  perfect  water  shed, 
preventing  it  from  obtaining  an  injurious  amount  of  the  fluid 
in  which  it  floats. 

The  epidermis  (i*i  upon,  and  gep^a  skin).  In  flowering 
plants,  the  epidermis  may  be  readily  perceived  to  be  a  mem- 
brane perfectly  distinct  from  the  cellular  and  fibrous  tissue 
which  it  covers,  on  account  of  the  magnitude  and  peculiar 
arrangement  of  its  cells.  The  epidermic  cells  contain  ordi- 
narily no  traces  of  chlorophyl,  and  therefore  the  epidermis  may 
be  readily  separated  from  the  parenchymatous  tissue,  with  which 
it  contracts  but  a  feeble  adhesion,  as  a  colorless  layer. 

The  epidermis  of  plants  is  clearly  intended  to  guard  their 
subjacent  vascular'  and  cellular  systems  from  injury,  to  pro- 


20  COMPOUND  ORGANS  OF  PLANTS. 

tect  those  systems  with  their  fluid  contents  against  changes  in 
the  state  of  the  atmosphere,  and  to  control  the  evaporation 
from  their  cells  within  proper  bounds. 

In  the  Lily  and  Balsam,  which  allow  of  ready  evaporation, 
the  epidermis  consists  of  a  single  layer  of  cells ;  but  in  plants 
which  inhabit  dry  situations,  it  is  so  constructed  as  to  retard 
evaporation,  and  either  consists  of  several  layers  of  cells,  as  in 
the  Oleander,  (Fig.  2,)  or  else  is  of  considerable  thickness,  as 
in  the  Aloe  and  Cactus.  By  this  provision  these  plants  are 
enabled  to  retain  their  moisture  for  a  greater  length  of  time. 

Fig.  2. 


Magnified  perpendicxilar  section  of  the  leaf  of  the  Oleander,  showing  the  thickness 
of  the  epidermis,  which  is  composed  of  three  layers  of  cells,  and  the  compact  vertical 
cells  of  the  upper  stratum  of  parenchyma. 

It  must  be  evident  that  the  exhalation  of  water  from  the 
leaves  is  to  a  certain  extent  necessary,  as  it  is  the  only  means 
by  which  the  sap  can  be  concentrated  and  rendered  subservient 
to  the  nutrition  of  the  plant.  Now  so  long  as  the  roots  can 
absorb  as  much  water  as  the  leaves  evaporate,  the  plant  will 
appear  fresh  and  green,  but  the  foliage  droops  (as  is  often  seen 
on  a  hot  summer's  day)  when  the  supply  at  the  roots  fails,  and 
there  is  too  much  evaporation  from  the  leaves. 

To  remedy  this  defect,  the  epidermal  surface  of  the  leaves  is 
furnished  with  self-acting  valves  or  openings,  called  stomata  or 
pores.  These  stomata  are  usually  of  an  oval  figure  with  a  slit 
in  the  middle,  and  are  so  situated  as  to  open  directly  into  the 


I 
THE   EPIDERMIS   AND   ITS   APPENDAGES.  21 

'  hollow  chambers  or  air-cavities  in  the  lower  stratum  of  paren- 
chyma. The  aqueous  contents  of  the  cells  of  the  parenchyma, 
throughout  the  whole  interior  of  the  leaf,  are  thus  brought  into 
immediate  contact  with  the  external  air,  whilst  at  the  same 
time  the  evaporation  of  their  contents  is  controlled  and  regu- 
lated by  these  foliar  apertures. 

This  is  done  in  the  following  manner.  The  slit  or  perfora- 
tion in  the  epidermal  surface  lies  between  two  cells,  which, 
unlike  the  rest  of  the  cuticular  cells,  generally  contain  some 
chlorophyll,  and  in  this  respect  resemble  the  parenchyma 
beneath.  These  cells  are  exceedingly  hygrometrical,  or  affected 
by  moisture.  When  the  atmosphere  is  damp,  these  two  cells 
become  swollen  and  turgid,  and  by  their  curvature  outwardly, 
open  the  orifice  and  allow  the  free  escape  of  the  superfluous 
water ;  but  when  the  atmosphere  is  dry,  they  straighten  and 
lie  parallel,  their  sides  being  brought  into  close  contact,  thus 
closing  the  aperture  and  stopping  evaporation  the  moment 
it  becomes  injurious  to  the  plant.  The  stomata  or  pores  of 
plants  are  therefore  analogous  to  the  governor  in  machinery, 
and  are  clearly  designed  to  regulate  the  operation  of  the 
vegetable  mechanism,  and  to  promote  the  healthy  passage  of 
fluids  through  the  system. 

The  structure  of  the  stomata  or  pores  of  plants,  may  be 
readily  perceived  on  the  epidermis  of  the  lily,  (Fig.  3,)  where 
they  are  unusually  large.  The  epidermis  -must  be  carefully 
removed,  and  having  been  freed  from  all  its  chlorophyll,  or 
green  matter,  it  must  be  placed  between  two  strips  of  glass 
with  a  drop  of  water  between  them  so  as  to  give  it  the  neces- 
sary degree  of  transparency.  Water  ought,  for  this  reason 
always  to  tie  used  whenever  objects  selected  from  the  tissues 
of  vegetables  are  examined  microscopically.  The  epidermis 

3 


22 


COMPOUND  ORGANS  OF  PLANTS. 


thus  prepared  will  exhibit  these  pores,  and  the  nature  and 
beauty  of  their  mechanism  will  be  seen  and  appreciated. 

The  stomata  are  generally  found  on  the  under  surface  of  the 
leaves,  the  mechanism  being  too  delicate  to  act  well  in  direct 

Fig.  3. 


Epidermis  of  the  lily,  showing  the  stomata  st,  composed  of  two  cells  with  an  o;  en- 
ing  or  slit  between  them. 

sunshine.  They  are  invariably  absent  from  the  parts  of  plants 
growing  beneath  the  water.  The  water-lilies  (Nuphar  and 
Nymphrea,)  and  all  plants  whose  leaves  float  on  the  water, 
have  the  stomata  on  the  upper  surface  of  their  leaves.  If  the 
leaves  of  plants  grow  erect,  the  stomata.  are  equally  distributed 
on  both  sides. 

Stomata  are  more  or  less  abundant  on  the  cuticle  of  all 
plants',  and  as  these  pores  perform  the  functions  of  exhalation 
in  proportion  to  their  number  on  different  plants,  it  is  neces- 
sary to  supply  them  with  water.  The  plant  called  Hydrangea 
quercifolia  has  on  one  square  inch  of  its  surface  160,000  pores, 


THE   EPIDERMIS   AND   ITS   APPENDAGES.  23 

and  therefore  requires  a  greater  supply  of  water  than  plants 
possessed  of  from  70  to  100  pores  on  the  same  superficies. 

The  rapidity  with  which  plants  wither  and  dry  when  not 
watered,  is  exactly  in  proportion  to  the  number  of  their  exhaling 
pores.  Thus  when  a  shower  of  rain  occurs  after  long  drought, 
our  readers  must  have  witnessed  that  many  plants  revive  long 
before  the  moisture  can  have  reached  their  roots.  The  only 
absorbents  in  this  case  were  the  stomata  on  the  epidermis. 

Occurring  on  the  surface  of  many  plants  are  Certain  minute 
expansions  of  the  epidermal  cells  termed  hairs.  These  consist 
either  of  a  single  elongated  cell,  or  of  several  cells,  placed  end 
to  end.  Those  hairs  which  are  not  connected  with  any 
peculiar  secretion,  are  termed  lymphatic.  Those,  on  the  other 
hand,  which  have  cellules  visibly  distended  at  their  base  or 
apex  into  receptacles  of  some  peculiar  fluid,  are  termed 
glandular. 

Fig.  4. 


Magnified  view  of  one  of  the  stinging  hairs  of  the  nettle  with  the  gland  at  its  base. 

It  is  from  these  secreting  hairs  that  the  beautiful  scent  of 
the  sweet  brier  is  derived,  and  the  sting  of  the  common  nettle 
(Fig.  4),  is  produced  by  an  acrid  fluid  ejected  through  its 
tubular  hairs  from  the  glandular  receptacles  at  their  base  a: 
Nettles  have  been  very  properly  termed  the  serpents  of  the 


24  COMPOUND  ORGANS  OP  PLANTS. 

vegetable  world ;  and  not  without  reason,  for  there  is  a 
remarkable  similarity  in  structure  between  the  poison  teeth  of 
the  latter  and  the  glandular  hairs  of  the  former.  In  both  the 
apparatus  is  tubular,  and  the  pressure  of  the  hair  or  tooth  on 
the  poison  gland  ejects  the  poison  into  the  system. 

The  poison  of  nettles  in  temperate  climates  is  not  of 
much  consequence,  but  as  we  approach  warmer  regions  sting- 
ing nettles  become  more  numerous  and  deadly.  "  Every 
person  is  acquainted  with  the  sting  of  the  common  nettle, 
Urtica  urens,  but  no  notion  can  be  formed  from  it  of  the 
torture  which  its  allies,  Urtica  stimulans,  Urtica  crenulata, 
produce  in  the  East  Indies.  A  gentle  touch  is  sufficient  to 
make  4he  limb  swell  up  with  the  most  fearful  rapidity,  and  the 
suffering  lasts  for  weeks ;  nay,  one  species,  growing  in  Timor, 
Urtica  urentissima,  is  called  by  the  natives  Daun  setan, 
DeviFs  Leaf,  because  the  pain  lasts  for  years,  and  sometimes 
death  itself  can  only  be  avoided  by  the  amputation  of  the 
injured  limb."* 

When  the  hairs  of  the  epidermis  are  hardened  by  deposits, 
as  in  the  rose  and  blackberry,  they  are  called  prickles,  (aculei). 
In  their  youth,  they  completely  resemble  hairs,  and  are 
dispersed  without  order  on  the  stem  and  leaves,  but  with 
age  they  become  thickened,  elongated  and  indurated,  as  may 
be  seen  on  the  rose,  where  they  present  themselves  in  every 
stage  of  development. 

Hairs  are  sometimes  attached  to  seeds  for  the  purpose  of 
scattering  them,  as  in  the  cotton  plant.  In  Rhus  cotinus,  or 
the  wig  tree,  the  flower  .stalks  are  changed  into  hairs. 

*  Dr.  Schleiden. 


THE   DIFFERENT   KINDS   OF    STEM.  25 

CHAPTER    II. 

THE.  DIFFERENT  KINDS  OF  STEM. 

THE  stem  may  be  regarded  as  that  portion  of  the  axophyte 
which  is  situated  between  its  two  extremities,  and  which  carries 
the  leaves  and  the  flowers. 

The  stem  exists  in  all  flowering  plants,  but  sometimes,  as  in 
Taraxacum  dens  leonis,  the  common  dandelion,  it  is  hardly 
developed  at  all,  so  that  the  leaves  and  even  the  floral  branches 
appear  to  spring  from  the  root.  These  plants  were  formerly 
considered  to  be  acaulescent  (a  without,  caulis  a  stem) ;  they 
have,  however,  a  true  stem,  but  it  is  so  contracted  in  its  growth 
as  to  be  hidden  in  the  earth. 

The  common  idea  that  all  the  subterranean  parts  of  plants 
are  roots  is  quite  erroneous.  The  production  of  buds  and 
leaves,  and  the  presence  of  leaf  scars,  are  the  distinguishing 
characteristics  of  the  stem,  and  the  following  roots,  so  called, 
which  exhibit  these  appearances,  are  only  its  subterranean 
modifications. 

The  rhizoma,  (/5t£a,  a  root).  This  stem  pursues  an  under- 
ground course,  growing  horizontally  at  a  depth  in  the  soil 
which  is  sufficient  to  protect  the  buds  on  its  surface,  from 
which  it  sends  forth  annually  herbaceous  branches  into  the 
air,  which  die  down  to  the  ground  at  the  close  of  the  flowering 
season.  In  Polygonatum  pubescens,  (fig.  5,)  the  annual  decay 
of  the  foliage  leaves  on  the  rhizoma  a  broad  and  conspicuous 
scar,  which  is  not  unlike  the  impression  of  a  seal,  and  for  this 
reason  the  plant  is  commonly  called  Solomon's  seal. 

3* 


26  COMPOUND    ORGANS   OF   PLANTS. 

Fig.  5. 


Rhizoma  of  Solomon's  Seal,  (Polygonatum  pubescens.) 

The  Thizoma  or  underground  stem  grows  after  the  manner 
of  ordinary  aerial  stems  by  the  development  of  both  lateral  and 
terminal  buds.  In  Polygonatum  pubescens  the  development  of 
the  subterranean  buds  is  alone  terminal,  but  in  other  perennial 
herbaceous  plants  the  lateral  as  well  as  terminal  buds  of  the 
rhizoma  are  developed,  and  the  subterranean  branches,  which 
are  called  suckers,  send  up  from  the  soil  aerial  stems.  These 
suckers  when  severed  from  the  parent  stem  and  planted  will 
grow  into  new  plants.  This  mode  of  multiplying  plants  is 
often  resorted  to  by  gardeners,  and  is  called  by  them,  propa- 
gating by  offshoots. 

The  bulb.  This  form  of  underground  stem  is  much  varied. 
It  may  be  a  scaly  bulb,  as  in  the  lily,  or  a  tunicated  bulb,  as 
in  the  onion.  These  varieties  of  bulbs  are  justly  regarded  by 
botanists  as  subterranean  buds  or  undeveloped  stems,  being  in 
every  respect  similar  to  the  ordinary  leaf  bud,  except  that  as 
they  grow  beneath  the  ground,  the  scales  or  imperfect  leaves 
which  envelope  them  are  more  thick  and  fleshy.  These  retain 


THE   DIFFERENT   KINDS   OF    STEM. 


27 


their  rudimentary  character  as  a  protective  covering  to  the 
inner  leaves,  which  grow  in  a  tuft  from  the  earth's  surface,  the 
flower  stem  rising  from  their  centre. 

In  the  tunicated  bulb  the  scales  enclose  each  other  in  a  con- 
centric manner,  each  scale  embracing  the  entire  circumference 
of  the  bulb.  The  outermost  scales  are  thin  and  dry,  the  inner- 
most thick  and  succulent.  Tunicated  bulbs  are  restricted  to 
such  plants  as  have  sheathing  leaves,  and  which,  consequently, 
embrace  at  their  base  the  entire  circumference  of  the  stem.  In 
the  scaly  bulb  the  scales  are  free  from  each  other  and  much 
smaller,  being  imbricated,  or  lying  one  on  the  other,  like  the 
tiles  of  a  house.  This  bulb  belongs  only  to  plants,  the  leaves 
of  which  are  sessile,  and  therefore  not  connected  with  the 
stem  by  a  sheathing  base. 


Fig.  6. 


Fig.  7. 


Fig.  6.— Bulb  of  the  garlic  with  a  crop  of  young  bulbs. 
Fig.  7.— Axillary  bulblets,  b,  of  Lilium  bulbiferum. 

Bulbs  being  subterranean  buds  or  undeveloped  stems,  give 
birth  to  new  buds  or  bulbs  in  the  axils  of  their  scales,  the 
rudimentary  leaf-like  nature  of  which  is  thus  rendered  apparent. 
The  young  bulbs  are  called  cloves.  This  mode  of  increase,  is 


28  COMPOUND   ORGANS   OF   PLANTS. 

exemplified  in  the  common  garlic,  (fig.  6.)  In  this  respect  the 
bulb  behaves  exactly  like  a  leaf-bud  after  it  has  been  lengthened 
into  a  branch.  One  or  more  of  these  young  bulbs  or  cloves 
may  develope  as  flowering  stems  the  next  season,  and  thus  the 
same  bulb  survives  and  blossoms  from  year  to  year. 

In  some  plants,  as  in  Lilium  bulbiferum,  (fig.  7,)  bulbs  are 
produced  on  the  stem  in  the  axils  of  the  leaves,  which,  when 
detached  from  the  stem  -and  placed  on  the  ground,  will  grow 
into  independent  plants.  These  bulbs  are  called  bulblets. 

The  tuber  is  a  subterranean  branch  which  is  arrested  in  its 
growth,  and  becomes  remarkably  thickened  in  the  place  of 
being  elongated.  It  is  seen  in  the  common  garden  potatoe, 
the  eyes  of  which  are  true  leaf  buds.  Hence  these  tubers 
when  cut  into  slices,  provided  the  slice  contains  an  eye,  will 
grow  and  become  independent  plants. 

In  the  lower  forms  of  their  development  stems  are  so  weak 
that  they  trail  along  the  ground,  never  rising  from  the  earth's 
surface.  *  In  other  instances  these  weak  stems  have  a  tendency 
to  grow  vertically ;  and  when  this  is  the  case  they  either  twine 
in  a  spiral  around  the  more  vigorous  herbage  in  their  vicinity, 
or  the  roots  of  the  phytons  take  a  horizontal  development  and 
exhibit  themselves  all  along  the  side  of  the  axophyte,  as  in  the 
Ivy  and  Virginian  creeper.  By  such  aerial  or  adventitious 
roots  such  plants  attach  themselves  to  the  surface  of  rocks 
and  the  bark  of  trees,  and  thus  elevate  themselves  to  the  air 
and  light. 

In  somje  plants,  such  as  the  pea  and  vine,  the  leaves  are 
developed  as  organs  of  support.  By  the  non-production  of  the 
parenchyma,  and  the  development  of  the  fibro-vascular  system, 
an  organ  called  a  tendril  is  produced,  which  has  a  tendency  to 
twine  round  any  body  with  which  it  may  come  in  contact. 


THE   DIFFERENT    KINDS    OF   STEM.  29 

The  tendrils  of  the  vine  and  pea  are  well  known ;  let  us 
show  the  beauty  of  their  mechanism.  When  the  plants  are 
young  they  are  put  forth  in  a  straight  line,  and  curved  into  a 
sort  of  hook  at  their  extremity.  In  this  manner  they  seem  as 
if  they  were  reaching  forward  for  the  purpose  of  catching  hold 
of  something  on  which  they  can  hang  for  support.  If  in  this 
state  a  young  twig  or  branch  be  borne  by  the  passing  breeze 
within  reach  of  their  hook,  they  immediately  catch  and  coil 
themselves  spirally  about  it.  Now  this  apparently  feeble 
organ  of  self-support  is  in  reality  a  powerful  instrument  of  self- 
defence,  and  the  storm  which  can  overpower  the  strength  of 
the  forest  trees,  prostrating  them  with  the  earth  as  it  rushes 
by  in  all  the  wildness  of  its  fury,  cannot  injure  these  plants. 
It  is  rendered  harmless  in  its  effects  by  the  elastic  yielding  of 
the  tendril,  which  thus  secures  these  weak  plants  from  being 
broken  off  from  the  object  to  which  they  have  attached  them- 
selves, and  from  sustaining  the  slightest  injury. 

In  the  grasses  the  stem,  which  is  hollow  and  fistular,  has 
received  the  name  of  culm  (culmus  a  straw).  This  structure 
also  prevails  in  other  plants,  and  is  a  beautiful  instance  of 
mechanical  contrivance  to  dispose  the  limited  quantity  of 
matter  in  the  stem  to  the  greatest  possible  advantage,  so  as  to 
give  the  greatest  strength  with  the  least  expenditure  of  mate- 
rial. By  this  hollow  cylindrical  disposition  of  the  matter,  an 
increase  of  strength  is  imparted  to  the  vegetable  structure 
equivalent  to  that  of  a  solid  stem  of  the  same  diameter.  The 
bones  of  animals  and  the  feathers  of  birds  are  tubes  or  hollow 
trunks,  combining  strength  with  lightness,  and  constructed  on 
the  same  principle. 

When  the  philosopher  Galileo  was  confined  in  the  dungeons 
of  the  Inquisition  for  teaching  the  heresy  of  the  motion  of  the 


30  COMPOUND   ORGANS   OF  PLANTS. 

earth,  he  was  visited  by  a  Catholic  priest,  who  accused  him  of 
Atheism.  ,The  persecuted  and  venerable  sage  met  the  accusa- 
tion by  the  following  beautiful  and  sublime,  though  simple  and 
affecting  appeal.  He  took  from  *the  floor  of  the  dungeon  on 
which  he  was  lying  a  wheaten  straw,  and  having  explained  the 
mechanical  and  scientific  principles  shown  in  the  structure  of 
the  stem,  told  the  priest  that  this  was  evidence  to  his  mind  of 
the  existence  of  a  G-od.  "  If,"  said  he,  « this  wheaten  straw, 
which  supports  an  ear  heavier  than  its  whole  stock,  were  made 
of  the  same  quantity  of  matter  disposed  in  a  solid  form,  it 
would  make  but  a  poor  thin  and  wiry  stem,  which  would  be 
snapped  with  the  slightest  breeze;  its  tubular  form  gives  it  the 
necessary  degree  of  strength,  and  preserves  it  from  destruc- 
tion." 

Not  only  the  strength  but  the  duration  of  stems  depends  on  the 
degree  of  their  development.  A  plant  is  considered  to  be  a 
herb  if  its  stem  invariably  dies  down  to  the  ground  each  year. 
Some  herbs  are  only  annuals  arriving  at  their  full  development 
and  the  term  of  their  existence  in  one  year,  the  act  of  repro- 
duction exhausting  their  vital  energies.  In  biennial  herbs  the 
whole  of  the  nutriment  assimilated  by  the  vegetative  organs 
the  first  year  is  consumed  by  the  act  of  reproduction  in  the 
second,  and  the  plant  necessarily  perishes.  In  herbaceous 
perennials  the  upper  part  of  the  plant  only  dies,  life  retrea  ts 
into  the  rhizoma,  and  with  the  return  of  light  and  heat  to  th  e 
earth  in  Spring,  the  plant  again  makes  its  appearance  above 
the  ground,  and  developes  into  its  wonted  figure  and  hues. 

The  same  species  may  become  an  annual,  biennial,  or  even  a 
perennial,  according  to  the  treatment  which  it  receives,  and 
the  circumstances  in  which  it  is  placed.  If  an  annual  plant  be 
deprived  of  its  flowers  and  preserved  from  the  inclemency  of 


THE    DIFFERENT    KINDS   OF   STEM.  31 

winter,  it  will  become  a  biennial.  On  the  other  hand,  tropical 
perennial  plants,  ^when  transported  into  temperate  climates 
become  annuals.  For  example;  The  beautiful  climbing  vine 
so  much  cultivated  called  (Cobaea  scandens,)  and  which  endures 
but  for  a  year  in  these  latitudes,  is  a  perennial  in  Chili  and 
Peru  its  native  country ;  so  also  the  castor  oil  plant,  (Ricinus 
communis),  which  in  Africa  forms  an  elevated  tree,  is  an 
annual  with  us. 

In  the  more  highly  developed  plants,  such'  as  shrubs  and 
forest  trees,  the  act  of  flov  ering  and  fruiting  consumes  only  the 
nutriment  enclosed  in  the  peduncle  and  its  immediate  supports, 
but  the  rest  of  the  plant  is  not  injured.  Yonder  leafless  tree, 
whose  branches  wave  in  the  winter's  wind,  loaded  with  snow, 
is  still  fraught  with  life.  All  along  its  central  axis,  in  its 
branches,  and  innumerable  branchlets  life  exists,  dormant 
beneath  the  scales  of  its  numerous  buds.  The  last  vegetative 
process  of  plants  with  ligneous  and  persistent  stems,  in  autumn, 
is,  in  fact,  the  formation  of  the  bud,  wherein  life  lies  dormant, 
yet  protected  from  the  severest  cold  of  winter  until  spring 
awakens  it  to  a  new  existence.  The  following  year  these  buds 
or  phytons  (^rov,  a  plant,)  as  they  have  been  correctly  namecl, 
develope  on  the  stem  or  axophyte,  and  from  them  ligneous 
matter  descends,  which  gives  an  additional  enlargement  and 
strength  to  the  vegetable  axis.  In  forest  trees,  therefore,  the 
stem  acquires  its  greatest  development.  A  forest  tree,  philoso- 
phically considered,  is  not  an  individual  plant,  as  is  commonly 
supposed,  but  a  community  of  individual  plants  growing 
together  about  a  common  vegetable  axis.  These  phytons  have 
a  downward  as  well  as  an  upward  development,  and  the  stem 
or  axophyte  is  formed  by  the  commingling  of  the  ligneous  or 
fibrous  matter  which  descends  from  them,  and  which  spreads 


32  COMPOUND  ORGANS  OF  PLANTS. 

in  the  earth  as  in  the  atmosphere  in  a  form  beautifully  appro- 
priate to  the  altered  condition  of  the  medium. 


CHAPTER   III. 

ON   THE    ROOT   OR   SUBTERRANEAN   APPENDAGES    OP   THE 
AXOPHYTE. 

THE  rhizoma  or  the  subterranean  part  of  the  vegetable  axis, 
has  appendages  like  the  aerial  part,  organically  adapted  to  the 
medium  in  which  they  are  developed.  These  appendages, 
emitted  by  the  rhizome  or  its  ramifications,'  are  ordinarily 
under  the  form  of  fibres  more  or  less  slender  and  delicate, 
commonly  cylindrical,  simple  or  branched,  called  radicle  fibres. 
It  is  the  assemblage  of  these  fibres  which  constitute  the  true 
root,  that  is  to  say,  the  organ  whose  function  is  to  draw  from 
the  soil  a  part  of  the  elements  necessary  to  the  life  and 
development  of  the  plant. 

Each  of  these  fibres  is  terminated  by  a  blunt  and  rounded 
extremity,  which  has  received  the  name  of  spongiole.  For  a 
long  time  this  was  considered  to  be  the  only  part  of  the  root 
which  absorbed  liquids.  But  it  is  now  ascertained  that  absorp- 
tion takes  place  throughout  the  whole  extent  of  the  radicle 
fibres,  the  centre  of  which  is  occupied  by  bundles  of  vessels. 

The- spongioles  or  spongelets  ought  not  to  be  reckoned  special 
organs.  Fig.  8  is  the  extremity  of  the  young  root  of  the 
sugar  maple,  (Acer  saccharinum,)  highly  magnified.  Now  the 
cellular  extremity  qf  the  root  or  the  spongiole,  a,  does  not  con- 
sist of  the  cells  most  recently  formed,  which  are  in  reality  an 


APPENDAGES   OF   THE   AXOPHYTE.  ,    33 

older  mass  of  cells,  pushed  forward  by  the  growth  of  the  cells 
at  b,  immediately  behind  them.  The  cells  of  the  point  consist 
of  older,  denser  tissues,  as  inspection  plainly  shows;  and  as 
these  decay  and  fall  away,  they  are  replaced  by  the  layer 
beneath.  The  point  of  all  root  fibres  is  capped  in  this  way. 

Fig.  8. 


It  would  appear  from  this  that  absorption  does  not  take  place 
to  any  considerable  extent  at  the  apex  of  the  root,  but  princi- 
pally through  the  more  recently  formed  tissues  behind  it,  and 
especially  by  those  capillary  cells  or  root  hairs  with  which  the 
surface  of  all  young  and  growing  roots  is  usually  covered. 
These  root  hairs  are  in  general  more  abundant  and  more 
developed  on  plants  growing  in  loose,  dry  sand.  Such  plants, 
in  order  to  obtain  as  much  moisture  as  possible  from  the  unfa- 
vorable element  in  which  they  are  placed,  shoot  forth  from 
every  fibre  an  incalculable  number  of  them. 

Roots  produce  radicle  fibres  and  root-hairs  instead  of  leaves, 
and  these  organs  like  leaves  are  deciduous  towards  autumn, 
being  annually  renewed  every  spring.  Hence  the  best  time 
for  transplanting  is  in  winter,  when  the  fibres  are  dead  or 
torpid,  or  in  early  spring  before  they  are  renewed.  Trans- 
planting after  the  season  of  growth  has  fully  commenced  is 
always  attended  with  more  or  less  injury  to  the  plant. 

4 


34  .         COMPOUND  ORGANS  OF  PLANTS. 

In  growing,  the  roots  of  plants  therefore  do  not  elongate 
through  their  entire  length,  but  increase  by  the  addition  of 
matter  to  their  advancing  points,  very  much  like  an  icicle, 
except  that  the  new  matter  is  added  from  within  and  not  from 
without.  Growing  in  a  medium  of  such  unequal  resistance 
as  the  soil,  they  elongated  through  their  entire  length  they 
would,  when  they  encountered  any  obstacle,  be  thrown  into 
knotted  and  contorted  forms,  which  would  prevent  their  acting 
as  conduits  of  food  from  the  soil,  which  is  their  peculiar  office. 
But  as  they  only  elongate  by  the  formation  of  fresh  tissue  at 
their  extremities,  they  are  thus  enabled  to  accommodate  them- 
selves to  the  nature  of  the  soil  in  which  they  grow ;  and, 
should  any  thing  impede  their  progress,  they  sustain  no  injury, 
but  following  the  surface  of  the  opposing  matter,  they  grow 
and  extend  themselves  until  they  again  enter  a  softer  and 
more  favorable  medium.  In  this  manner  they  penetrate  the 
soil,  as  it  were,  in  search  of  food,  insinuating  themselves  into 
the  minutest  crevices  of  rocks,  and  extending  themselves  from 
place  to  place,  as  the  nutriment  in  their  own  immediate 
neighborhood  is  consumed. 

Now  all  newly  formed  vegetable  is  extremely  hygrometrical, 
and  hence  absorption  takes  place  throughout  the  whole  extent 
of  the  newly  formed  tissue. 

The  law  which  regulates  this  absorption  has  been  recently 
discovered  by  M.  Dutrochet,  a  distinguished  French  physiolo- 
gist. It  is  this :  if  two  fluids  of  unequal  densities  be  separated 
by  an  animal  or  vegetable  membrane,  the  denser  fluid  will 
draw  the  lighter  through  the  membrane  with  a  force  propor- 
tional to  the  difference  of  density  of  the  two  fluids.  A  simple 
experiment  will  illustrate  this.  (Fig.  9). 

Take  a  short  tube,  and  cover  one  end  with  a  piece  of  blad- 


APPENDAGES   OP   THE   AXOPHYTE.  35 

der ;  partly  fill  the  tube  with  a  strong  solution  of  sugar,  and 
immerse  it  in  a  vessel  containing  water.  In  an  hour,  or  more, 
the  denser  fluid  will  be  found  to  have  attracted  the  water 
through  the  membrane  and  to  have  risen  considerably  in  the 
tube.  This  property  is  called  Endosmosis,  (SV&QV,  inwards, 
I  seek.) 

Fig.  9. 


Now  the  cells  of  the  roots  and  the  entire  system  of  the 
plant,  owing  to  the  evaporation  of  water  from  the  leaves,  always 
contain  a  fluid  dense  and  concentrated.  The  water  in  the 
earth  is  therefore  attracted  into  the  plant  by  means  of  the 
denser  fluid  contained  in  the  cells  of  the  root, — or  in  other 
words,  it  enters  the  plant  by  endosmosis. 

This  simple  endosmotic  law  pervades  all  vitally  active  and 
newly  formed  vegetable  tissues,  and  seems  to  be  the  only  cause 
of  all  the  remarkable  movements  of  roots.  For  example,  it  is 
well  known  that  roots  will  turn  aside  from  a  barren  for  a 
fertile  soil,  so  that  to  stop  their  growth  in  any  given  direction, 


36  COMPOUND  ORGANS  OP  PLANTS. 

it  is  only  necessary  to  interpose  a  trench  of  gravel  or  sand  between  . 
them  and  the  premises  they  are  forbidden.  How  is  this  to  be 
accounted  for  ?  Are  we  to  suppose,  as  some  have  done,  a  sort 
of  prescience  on  the  part  of  the  vegetable  ?  On  the  contrary, 
is  it  not  all  clearly  explicable  on  the  principle  of  endosmosis  ? 
There  is  always,  in  the  forming  and  vitally  active  cells  at  the 
extremities  of  the  roots,  a  thicker  fluid  than  the  fluid  in  earth ; 
the  fluid  in  the  earth  is  attracted  by  endosmosis  through  the 
cell  walls  into  the  system  of  the  plant,  and  becoming  assimi- 
lated, the  newly  formed  cells  of  the  roots  necessarily  take  the 
direction  of  the  most  fertile  and  favorable  soil. 

Roots  developing  in-  the  soil  have  a  natural  tendency  to  an 
avoidance  of  the  light,  whilst  the  stem  and  leaves  seem  to  seek 
for  the  same.  Hence  hyacinth  bulbs  will  grow  much  better 
in  water-glasses  which  are  of  a  dark  color,  than  in  white 
uncolored  ones.  So  also  when  Dutrochet  caused  a  misseltoe 
seed  to  germinate  on  the  inside  of  a  window  pane,  it  sent  its 
roots  inwards  towards  the  apartment;  when  on  the  outside  of 
the  pane  it  did  the  same.  Hence  when  seed  is  sown  by 
nature  or  the  hand  of  art,  however  the  seed  may  fall,  yet  in 
germination  the  radicle  so  bends  itself  as  to  sink  perpendicu- 
larly into  the  soil,  whilst  the  stem  rises  perpendicularly 
from  it. 

The  force  with  which  the  radicle  or  root  descends  is  very 
considerable,  and  many  attempts  have  been  made  to  change  its 
obstinate  tendency  to  burrow  in  the  ground,  but  without  effect. 
We  know  not  yet  the  cause  of  this  invincible  tendency  of  the 
radicle  towards  the  earth's  centre.  It  has  been  thought  that 
the  humidity  which  exists  in  greater  abundance  in  the  soil 
exercises  a  sort  of  attraction  on  the  radicle,  but  Duhamel  has 
shown  that  it  is  not  so.  He  caused  seeds  to  germinate  between 


APPENDAGES   OP    THE   AXOPHYTE.  37 

two  sponges,  saturated  with  water,  brought  near  each  other, 
and  suspended  in  the  air  by  means  of  a  double  thread.  When 
germination  was  sufficiently  advanced,  the  radicles  instead  of 
bearing  to  the  right  and  left  towards  the  water  in  the  sponges', 
glided  between  them,  so  that  they  ultimately  hung  below  them 
into  the  atmosphere.  It  is  not  then  the  humidity  in  the  soil 
which  causes  the  radicles  to  penetrate  its  surface.  It  would 
rather  seem  that  this  tendency  ought  to  be  attributed  to  a 
particular  force,  which  developes  a  sort  of  polarity  at  the 
period  of  germination,  which  produces  an  opposition  of  growth 
in  the  two  extremities  of  the  vegetable  embryo,  causing  the 
plumule  to  rise  towards  the  zenith,  and  the  radicle  to  move  in 
the  direction  of  the  earth's  centre. 

The  radicle  fibres  generally  spring  from  the  subterranean 
portions  of  the  axophyte,  but  the  aerial  portions  of  that  organ 
are  equally  capable  of  emitting  them.  When  this  is  the  case 
they  are  designated  under  the  name  of  aerial  or  adventitious 
roots.  Some  woody  vines,  as  the  Bignonia  or  Trumpet-creeper, 
the  Rhus  toxicodendron  or  Poison  ivy,  and  the  Hedera  helix 
or  European  ivy,  climb  by  aerial  rootlets,  in  which  way  they 
reach  the  summits  of  the  tallest  trees,  and  loftiest  buildings, 
giving  beauty  even  to  the  mouldering  ruin.  Such  plants, 
however,  derive  their  nutriment  from  their  ordinary  roots 
embedded  in  the  soil,  their  copious  aerial  rootlets  merely  serv- 
ing them  for  mechanical  support.  The  tenacity  with  which 
these  aerial  rootlets  adhere  to  trees,  rocks,  and  even  to  the 
hardest  flint,  is  truly  astonishing ;  and  the  height  to  which  the 
plants  themselves  will  ascend,  seems  to  cease  only  because 
they  can  find  nothing  higher  on  which  they  can  support  them- 
selves. In  warm  climates  these  twining  plants  (lianas)  take 
a  much  higher  degree  of  development ;  their  stems  are  ligneous, 

4* 


38  COMPOUND  ORGANS  OF  PLANTS. 

persistent,  and  sometimes  very  thick,  whilst  with  us  they  are 
very  slender  and  herbaceous  and  perish  annually.  Heat  and 
humidity  are  powerful  agents  in  promoting  vegetation,  and 
hence  its  superior  activity  in  the  tropics. 

The  roots  emitted  "by  the  aerial  portion  of  the  axophyte 
sometimes  remain  free  and  floating  in  the  atmosphere,  and  . 
sometimes  they  descend  as  far  as  the  soil,  which  they  penetrate 
in  order  to  draw  from  it  additional  nourishment.  These  pecu- 
liarities are  observable  in  many  of  the  vegetables  of  the  warm 
and  sunny  South.  A  great  many  palms,  figs,  and  orchideous 
plants  develope  these  roots. 

When  the  roots  continue  aerial,  the  plants  are  termed 
epiphytes  (Hfti  upon,  and  tywtov  plant).  They  are  so  called 
because  they  grow  to  other  plants  as  mere  points  of  attach- 
ment, dropping  their  roots  into  the  atmosphere  from  which 
they  derive  all  their  food,  and  which  always  continue  aerial- 
and  greenish,  and  to  distinguish  them  from  parasites,  which 
obtain  their  nutriment  from  the  plant  on  which  they  are  found. 
These  plants  abound  in  the  tropical  forests  of  South  America, 
which  they  enrich  and  beautify  by  their  gorgeous  and  fragrant 
flowers.  That  the  trees  on  which  they  grow  are  mere  points 
of  attachment,  and  not  sources  of  food,  is  evident  from  the  fact 
that  they  may  be  attached  to  any  substance  whatever,  as  for 
instance  to  the  rafters  of  the  stove  or  hot  house,  where  they 
will  grow  with  an  equal  amount  of  vigor  and  luxuriance. 

The  roots  of  the  epiphytes  or  air-plants  as  naturally  avoid  the 
ground  and  darkness  as  the  roots  of  other  plants  seek  for  the 
same ;  they  require  air  and  light,  and  may  be  seen  searching 
for  it  in  the  warm,  moist  atmosphere  of  the  conservatories, 
through  the  crevices  of  the  baskets  filled  with  chips  and  char- 
coal in  which  they  are  generally  kept.  In  other  instances, 


APPENDAGES   OF   THE   AXOPHYTE.  39 

these  aerial  roots  emitted  from  the  stem  into  the  open  air, 
descend  to  the  ground,  and  establish  themselves  in  the  soil. 
Many  plants  of  tropical  climates  present  this  phenomena. 
Amongst  which  we  may  mention  the  Ficus  religiosa,  or 

Fig.  10. 


An  Epiphytic  orchid  (Maxillaria)  of  warm  climates. 

Banyan  tree  of  British  India.  This  tree  drops  from  its  hori- 
zontal branches,  roots  into  the  air,  which,  swinging  in  the 
breeze  like  pendant  cords,  do  finally  reach  the  soil,  into  which 
they  penetrate,  when  they  become  metamorphosed  or  changed 
into  stems,  and  increasing  in  diameter,  give  nutriment  and  a 
natural  support  or  prop  to  the  heavy  branches  from  which 
they  originally  descended,  so  that  those  branches  can  extend 
laterally  still  farther  from  their  parent  trunk.  By  numerous 
growths  of  this  kind,  one  tree  ultimately  becomes  the  centre  of 
a  family  forest,  their  united  branches  and  foliage  spreading 
over  a  considerable  extent  of  ground. 

The  Pandanus  or  Screw  pine  may  be  cited  as  another 
instance.  In  this  case,  when  the  tree  is  much  exposed  to  the 
powerful  winds  of  the  tropics,  strong  roots  are  emitted  from 
the  lower  part  of  the  main  trunk,  which,  striking  into  the  soil, 


40  COMPOUND  ORGANS  OF  PLANTS. 

act  as  props  to  the  stem,  giving  the  tree  the  appearance  of 
having  been  raised  from  the  ground.  If,  however,  the  tree  is 
under  shelter,  or  cultivated  in  a  stove  or  hot-house,  those 
thick,  strong  roots  or  props,  provided  by  nature,  do  not 
develop!,  but  are  still  seen  as  protuberances  on  the  surface  of 
the  stem.  The  same  phenomena  is  perceptible  on  a  small 
scale  in  the  stern  of  the  Zea  mays,  or  Indian  corn,  the  lower 
joints  of  which  give  forth  aerial  rootlets,  which  reach  or  do 
not  reach  the  soil,  according  to  the  amount  of  support  required 
by  the  plant. 

In  general  it  may  be  remarked,  that  these  adventitious  roots 
are  developed  from  those  parts  of  the  stem  where  the  nutritive 
sap  encounters  some  obstacle  to  its  free  circulation,  and  in  par- 
ticular at  the  nodes  or  accidental  nodosities  which  exist  on  the 
stem  or  its  branches. 

We  are  able,  whenever  we  please,  to  produce  these  adventi- 
tious roots  on  the  young  branches  of  most  ligneous  vegetables. 
It  is  only  necessary  to  surround  the  young  branch  with  humid 
parth  contained  in  any  kind  of  pot  or  vase.  At  the  end  of  a 
definite  period,  varying  according  to  the  species^  the  roots  will 
develop^  themselves,  and  the  young  branch  can  be  separated 
and  will  form  another  plant.  This  is  a  mode  of  multiplication 
very  useful  in  horticulture. 

We  see  the  same  results  produced  continually  in  nature 
under  similar  circumstances.  Most  creeping  stems  produce 
roots  &t  every  leaf  node,  that  is  to  say,  when  there  are  the 
suitable  conditions,  moisture,  a  certain  amount  of  shade,  and 
immediate  contact  with  the  earth ;  and  the  branches  of  such 
stems  as  are  vertical,  if  bent  to  the  ground  and  covered  with 
earth,  almost  always  take  root.  This  is  sometimes  done  by 
gardeners,  who  bury  the  limbs  of  shrubs  by  bending  down  the 


APPENDAGES   OF   THE   AXOPHYTE.  41 

body  of  the  tree,  after  which  each  limb,  being  severed  from  the 
parent,  forms  a  new  tree. 

Separate  pieces  of  young  stems  containing  a  bud,  and  called 
by  gardeners  cuttings,  will  also  take  root  if  due  care  be  taken 
with  them.  For  a  tree  is  not  an  individual,  as  is  commonly 
supposed,  but  a  collection  of  individuals,  an  elongation  of  indi- 
vidual buds,  which,  in  their  development  into  branches,  live 
on  their  parent  stem,  into  which  they  send  down  roots  just  as 
that  parent  stem  itself  sends  its  roots  into  the  soil.  For  this 
reason  the  bud  of  one  plant  may  be  transferred  to  the  stem  of 
a  similar  or  nearly  related  species,  to  which,  if  it  be  carefully 
fitted,  it  will  soon  become  rooted  and  develope  into  a  branch, 
being  sustained  by  the  stem,  into  which  it  has  been  engrafted 
equally  with  the  natural  branches  of  the  tree. 

The  observations  of  Mohl  and  linger,  both  eminent  physio- 
logists, have  proved  that  adventitious  or  aerial  roots  are  all 
formed  in  a  very  similar  manner.  They  show  themselves  at 
first  under  the  form  of  a  little  conical  excrescence  or  tubercle, 
the  base  of  which  rests  on  the  wood.  As  they  increase,  they 
turn  aside  the  cells  of  the  tuber  and  cortical  parenchyma  which 
they  traverse,  and  finally  form  a  slight  prominence  under  the 
epidermis.  A  little  later,  the  epidermis  is  torn  in  a  direction 
parallel  to  the  axis  or  stem,  and  the  root  shows  itself  at  the 
exterior  and  directs  itself  towards  the  soil. 

The  roots  of  plants  generally  bury  themselves  in  the  soil, 
but  some  plants  are  parasites  (rfapa  beside,  altos  food),  or 
derive  their  nutriment  from  the  plants  on  which  they  grow 
and  into  which  they  fix  their  roots,  and  cannot  therefore  be 
cultivated  on  the  ground.  Some  parasites  grow  on  the  roots 
of  trees,  as  the  Epiphegus  Virginiana,  or  beech  drops,  which  is 
found  beneath  the  shade  of  beech  trees,  and  on  the  roots  of 


42  COMPOUND  ORGANS  OF  PLANTS. 

which  it  is  parasitic ;  others  attach  themselves  to  the  stem  and 
branches  of  trees,  as  the  cuscuta  or  dodder  and  Viscum  fla- 
vescens  or  mistletoe. 

The  Indian  pipe  (Monotropa  uniflora)  is  one  of  the  most 
remarkable  of  our  native  parasites.  This  plant  may  be  found 
occasionally  in  the  deep,  rich  woods  of  North  America,  during 
the  summer  months.  It  is  a  singularly  pallid  and  fungous- 
looking  plant,  to  which  order  it  seems  to  approximate  not  only 
in  appearance,  but  also  in  the  exercise  of  its  functions.  It  is 
fleshy,  scentless,  and  snow-white  throughout,  and  rises  to  a 
height  of  from  four  to  eight  inches  above  the  ground,  bearing 
at  its  summit  a  solitary  terminal  flower,  which  is  at  first 
drooping,  and  in  this  state  the  plant  looks  not  unlike  a  pipe  in 
appearance,  but  afterwards  becomes  erect.  The  whole  of  the 
plant  turns  black  in  drying. 

The  roots  of  many  species  of  plants  are  not  fixed  to  any  sub- 
stance whatever,  the  plant  possessing  a  sort  of  locomotive 
power.  This  is  the  case  with  several  kinds  of  aquatic  plants, 
as  the  Lemna,  or  duckmeat,  a  little  frond-like  plant,  which 
covers  the  surface  of  stagnant  pools  with  its  scum-like  vegeta- 
tion, and  drops  its  little  filiform  roots  into  the  water,  on  the 
surface  of  which  it  floats.  So  also  the  Fucus  natans,  a  species 
of  marine  algae,  is  found  in  the  Gulf  of  Florida  and  other 
parts  of  the  ocean,  floating  many  hundreds  of  miles  away 
from  land.  This  plant  has  no  distinct  root,  and  is  of  course 
found  only  within  certain  latitudes. 

Functions  of  roots. — The  principal  function  of  roots,  con- 
sists in  drawing  from  the  earth,  or  from  any  other  medium 
in  the  midst  of  which  they  are  plunged,  those  substances  which 
serve  for  the  nutrition  of  the  plant.  Their  organization  and 
vital  phenomena  prove  this. 


APPENDAGES   OF   THE   AXOPHYTE.  43 

M.  Macaire  has  proved  that  plants  possess  the  power  of 
excreting  by  their  roots  such  injurious  matters  as  they  may 
occasionally  necessarily  meet  with  in  the  soil,  and  absorb 
from  it  in  the  progress  of  their  development  under  ground. 
This  gentleman  took  a  fibrous-rooted  plant,  and  having  sepa- 
rated its  fibres  into  two  sets,  he  placed  one  set  in  a  glass  con- 
taining distilled  water,  and  the  other  in  a  solution  of  acetate 
of  lead.  After  a  few  days  he  found  that  the  fibres  dipping  into 
the  solution  of  acetate  of  lead,  had  taken  up  that  poison  into 
the  plant,  but  that  the  same  poison  had  been  excreted  or 
thrown  out  by  the  other  set  of  fibres  into  the  glass  containing 
distilled  water.  Fo  r  on  applying  sulphuretted  hydrogen,  the 
test  for  the  acetate,  he  found  the  distilled  water  was  impreg- 
nated with  it. 

In  this  experiment  we  see  that  the  poison  was  forced  into 
the  circulatory  system  of  the  plant,  which  induced  a  self-pre- 
servative effort  on  its  part  analogous  to  that  made  in  the 
higher  forms  of  life.  All  the  experiments  made  on  plants 
with  narcotics  and  other  poisons  prove  that  they  possess  a 
principle  of  life  analogous  to  that  of  animals. 

The  roots  of  plants  when  developed  in  the  soil,  are  also 
clearly  designed  to  fix  them  in  an  upright  position,  so  as  to 
prevent  them  from  being  overturned  by  animals,  by  the  force 
of  the  winds,  or  by  any  other  cause.  Hence  it  is  that  the 
roots  ,of  a  tree  are  always  most  numerous  and  strong  to.  wind- 
ward, or  in  the  direction  of  the  prevailing  winds.  When  the 
tree  is  sheltered  on  every  side,  there  is  little  lateral  extension 
of  its  roots,  and  they  naturally  develope  downwards  into  the 
earth.  So  also  the  roots  of  trees  growing  on  the  sides  of  roads 
or  the  banks  of  rivers,  will  curve  into  the  embankment,  and 
thus  prevent  the  tree  from  being  undermined  or  "washed  away. 


44  COMPOUND  ORGANS  OF  PLANTS. 

The  roots  of  rock  plants  adhere  to  their  surface  and  crevices 
with  the  most  astonishing  tenacity ;  as  for  example,  the  beau- 
tiful wild  columbine,  (Aquilegia  Canadensis,)  one  of  the  early 
spring  flowers  of  the  northern  States. 

The  roots  of  plants,  particularly,  the  fibrous  and  matted 
roots  of  the  sedge  and  grass  tribe,  bind  together  the  loose  soil 
on  the  sea-shore,  and  prevent  it  from  drifting  inland.  •  On 
many  coasts,  the  inward  drift  of  the  sand  by  the  strong  sea 
breezes  which  prevail,  produces  hills  of  sand  called  dunes. 
The  safety  of  these  shores  is  greatly  promoted  by  a  species  of 
grass  called  theArundo  arenaria,  whose  thick  and  matted  roots 
bind  together  the  loose  sand  and  prevent  its  desolating  effects. 

That  disintegration  and  destruction  of  rocks  mechanically  and 
chemically,  which  is  continually  going  forward  in  nature,  is 
also  prevented  from  being  carried  forward  to  an  injurious 
extent  by  the  fibrous  roots  of  grasses  and  other  plants. 

It  is  a  fact  well  known  to  practical  geologists,  that  when 
rocks  rise  above  the  surface  of  the  earth  in  cliffs  and  ridges, 
they  become  exposed  to  the  mechanical  and  chemical  action  of 
the  atmosphere,  and  their  surface  gradually  shivers  off,  crumbles 
down,  and  wears  away.  Hence  loose  matter  collects  at  the 
bottom  of  the  escarpment,  forming  in  the  course  of  ages,  a 
slope  of  disintegrated  material,  called  by  geologists  a  talus. 
The  process  of  disintegration  continues  until  the  talus  of  fallen 
fragments  has  accumulated  to  the  very  summit  of  the  escarp- 
ment, so  as  to  hide  it  altogether.  Now  so  long  as  the  face  of 
the  escarpment  is  exposed,  and  the  fall  of  the  detached  frag- 
ments continues,  vegetation  will  not  seize  on  the  slope ;  but 
when  the  disintegrated  material  has  acquired  that  degree  of 
sloping,  which  is  called  by  geologists,  the  angle  of  repose,  or 
has  accumulated  to  the  very  summit  of  the  escarpment,  no 


APPENDAGES   OF    THE    AXOPHYTE.  45 

more  fragments  will  roll  down,  and  vegetation  will  cover  the 
slope.  Vegetation  appears  on  no  soil  but  what  is  in  a  state  of 
rest,  and  when  it  is  once  established  in  any  place,  it  is  not  only 
a  sure  indication  that  the  soil  is  at  rest,  but  a  means  of  keeping 
it  so.  It  is  by  operations  of  this  kind,  not  performed  in  a  day, 
but  in  ages,  that  rugged  peaks  and  abrupt  precipices  are 
gradually  transformed  into  rounded  summits,  gentle  slopes, 
and  habitable  surfaces.  On  precisely  the  same  principle,  the 
sloping  sides  of  railways  are  secured  from  disintegration  and 
destruction,  by  being  sown  with  grass  seeds  or  covered  with 
grass  sods. 

The  lower  orders  of  the  Cryptogamia,  or  flowerless  plants, 
such  as  lichens  and  mosses,  appear  to  derive  their  nutriment 
mainly  from  the  atmosphere.  Mosses  appear  to  take  in  their 
nutriment  from  the  air  by  their  whole  expanded  surface, 
although  doubtless  the  delicate  root  hairs  below  that  surface 
perform  their  part  in  absorption.  Hence  some  species  are  only 
found  growing  on  the  bark  of  trees,  others  on  rocks  and 
boulders,  whilst  numerous  genera  cover  the  surface  of  the 
ground.  The  roots  of  lichens,  when  they  have  any,  are  mere 
holdfasts,  the  plant  being  developed  wholly  from  the  atmo- 
sphere. Some  species,  however,  would  seem  to  attach  them- 
selves to  stones  of  a  calcareous,  whilst  others  form  a  beautiful 
plaiting  on  the  surface  of  whins,  sandstones  and  granites. 
These  atmospheric  cryptogamia,  are  the  first  plants  which 
clothe  the  surface  of  barren  rocks,  and  by  their  decay  form 
a  humus  or  foothold  for  a  more  highly  organized  vegetation. 


46  COMPOUND  ORGANS  OP  PLANTS. 

CHAPTER   IV. 

ON  THE  ORGANIZATION  OF  THE  STEM. 

WHEN  we  examine  anatomically  the  stems  of  phanerogamous 
plants,  we  find  them  to  be  remarkably  similar  in  their  internal 
structure.  The  stems  of  forest  trees,  ligneous  and  persistent 
for  centuries,  differ  only  from  the  stems  of  the  herbaceous  and 
humble  plants  which  grow  beneath  their  shade,  in  the  degree 
of  their  development;  they  are  constructed  on  precisely  the 
same  plan,  and  in  all  the  varieties  of  their  growth,  for  the  most 
part,  are  reducible  to  either  one  or  the  other  of  the  two  follow- 
ing forms  of  vegetable  organization. 

The  exogenous  (££«,  outward,  and  yewdsw,  to  produce,)  or 
outside-growing  stem,  so  called,  because  this  kind  of  stem 
increases  in  diameter  by  successive  annual  additions  of  bun- 
dles of  vascular  and  fibrous  tissue  to  its  outside.  Such  a  stem 
exhibits  on  the  cross-section  a  number  of  concentric  tongs  of 
wood,  which  mark  the  successive  annual  growths  of  the  tree, 
surrounding  a  central  column  of  pith,  the  whole  enclosed 
by  a  hollow  cylinder  of  bark.  The  forest  trees  of  the  northern 
United  States,  and  the  major  part  of  our  herbaceous  plants, 
are  all  constructed  on  this  plan ;  and  the  cross-sections  of  an 
oak  branch,  or  of  any  other  tree,  will  show  the  rings,  and  the 
nature  of  an  exogenous  stem. 

The  endogenous  (evSov,  within,)  or  inside-growing  stem,  so 
called,  because  it  increases  in  diameter  by  successive  additions 
of  fibro-vascular  and  cellular  matter  to  its  inside.  The  growth 
of  these  plants  is  carried  on  by  means  of  the  thick  cluster  of 


ORGANIZATION    OF   THE    STEM.  47 

leaves  with  which  they  are  terminated  superiorly,  and  from 
them  the  new  vegetable  matter  descends  along  the  centre  of 
the  stem,  and  pushes  outward  the  parts  first  formed. 

The  oldest  and  hardest  part  of  the  stem  of  an  endogen  is 
that  nearest  the  circumference ;  for  the  more  the  external  parts 
are  pressed  by  the  descent  internally  of  new  vegetable  matter, 
the  denser  must  they  necessarily  become.  It  is  owing  to 
this  external  hardness  that  many  endogenous  plants  have  no 
lateral  buds  or  branches,  because  they  are  unable  to  penetrate 
the  hard  casing  of  the  stem. 

On  the  cross-section,  the  stem  of  an  endogen  is  not  distin- 
guishable into  bark,  wood,  and  pith,  neither  does  it  present 
any  appearance  of  concentric  rings;  for  the  stem  of  an  endo- 
gen is  composed  of  separate  bundles  of  vascular  or  woody 
tissue  irregularly  imbedded  in  a  mass  of  cellular  tissue,  which 
bundles  are  distinctly  traceable  down  into  the  stem  from  the 
base  of  the  leaves  at  its  summit,  and  then  curving  outwards 
they  generally  terminate  in  the  bark.  Hence  on  the  cross- 
section  the  cut  ends  of  these  bundles  are  visible  in  the  form 
of  dots,  interspersed  through  the  uniform  cellular  tissue,  with- 
out any  apparent  order,  although  more  commonly  crowded 
towards  the  circumference.  Hence,  also,  we  see  the  reason 
why  the  bark  of  an  endogen  is  inseparable  from  the  rest  of  the 
stem  without  a  laceration  of  its  fibres. 

The  plants  whose  structure  is  endogenous  in  the  northern 
United  States  are  few,  and,  with  the  exception  of  the  green 
brier,  entirely  herbaceous.  The  grasses,  the  iris,  the  Indian 
corn,  are  humble  representatives  of  endogenous  plants,  which 
attain  their  full  development  and  display  their  noble  arborescent 
forms  only  under  the  influence  of  a  tropical  sun.  The  palms, 
screw  pines,  plantains,  and  bananas  of  the  tropics,  are  all  endo- 


48  COMPOUND   ORGANS   OF   PLANTS. 

genous,  and  present  a  striking  contrast  to  the  exogens  of  tem- 
perate latitudes.  A  tall,  cylindrical  and  unbranched  stem  rises 
to  the  height  of  from  100  to  150  feet,  crowned  at  the  summit 
with  a  magnificent  cluster  of  leaves,  many  feet  in  length,  bend- 
ing elegantly  downwards,  and  presenting  altogether  one  of  the 
most  graceful  and  beautiful  objects  that  can  adorn  a  landscape. 

The  Exogenous  Stem. — Since  the  exogenous  class  of  plants 
is  by  far  the  largest  in  every  part  of  the  world,  and  embraces 
all  the  trees  and  shrubs  with  which  we  are  familiar  in  northern 
climates,  the  structure  of  this  kind  of  stem  demands  a  more 
detailed  and  particular  investigation.  Every  exogenous  stem 
presents,  on  the  cross-section,  an  arrangement  of  matter  into 
three  parts,  called,  respectively,  the  bark,  the  wood,  and  the 
pith.  To  obtain,  however,  a  clear  idea  of  the  origin  of 
the  exogenous  stems,  it  is  necessary  to  follow  the  course  of 
the  development  of  the  stem  from  the  embryo  state. 

The  first  year's  growth. — If  we  place  a  seed  in  the  ground 
at  the  temperature  of  32°  Fahr.,  it  will  remain  inactive  until 
it  finally  decays;  but,  if  the  earth  be  moist  and  above  the 
temperature  of  32°,  and  the  seed  be  effectually  screened  from 
the  action  of  the  light,  its  integuments  will  gradually  imbibe 
moisture,  soften,  and  swell,  oxygen  will  be  absorbed,  carbonic 
acid  expelled,  and  the  vital  action  of  the  embryo  will  com- 
mence. It  now  elongates  downwards  into  the  earth  by  its 
radicle,  and  upwards  into  the  air  by  its  plumule,  or  young 
stem,  lifting  the  cotyledons  above  the  earth 's  surface.  The 
cotyledons  thus  elevated  acquire  a  green  color,  by  the  deposi- 
tion of  carbon  absorbed  from  the  atmosphere  under  the  influ- 
ence of  solar  light,  and  ultimately  assume  the  form  of  two 
opposite  leaves.  The  process  of  germination  is  now  completed, 
and  the  root,  stem  and  leaves  being  formed,  we  have  a  simple 


ORGANIZATION    OF   THE    STEM.  49 

plant  perfect  in  all  its  parts  and  dependent  for  its  future 
growth  and  sustenance  on  its  leaves  and  roots. 

Let  us  now  examine  the  successive  modifications  of  the  inter- 
nal structure,  from  the  commencement  of  germination  to  the 
growth  of  the  first  pair  of  leaves  and  the  completion  of  this 
the  first  stage  of  vegetation.  At  first  the  embryo  consists 
wholly  of  cellular  tissue;  as  soon,  however,  as  it  begins  to 
grow,  even  while  the  cotyledons  only  are  developing,  some  of 
the  cells  begin  to  elongate  into  tubes  longitudinally,  assuming 
the  form  of  vascular  and  woody  fibre.  These  nascent  wood-cells 
extend  upwards  into  the  cotyledons,  and  downwards  into  the 
radicle,  and  are  finally  seen  in  a  cylindrical  form  in  the  centre 
of  the  stem.  This  longitudinal  elongation  of  the  cells  does  not 
take  place  at  random,  but  certain  determinate  cells  only 
thus  change  their  character,  whilst  the  others  wholly  retain 
or  depart  but  slightly  from  their  primitive  form.  A  horizontal 
section  of  the  plumule  at  this  stage  of  development  will  show 
this. 

The  sap  elaborated  in  the  first  pair  of  leaves  contributes  to 
the  upward  growth  of  the  plumule  or  young  stem,  and  to  the 
development  of  the  second  pair  of  leaves ;  the  new  wood-cells 
extend  through  them  to  form  their  frame-work,  making  the 
woody  stratum  in  the  second  internode  as  it  lengthens,  and  con- 
tributing at  the  same  time  to  the  increase  of  the  stem  beneath 
them;  and  the  same  process  is  repeated  throughout  the  whole 
growth  of  the  season  with  every  fresh  development  of  leaves. 

The  woody  fibre  having  rapidly  increased,  ascending  and 
descending  the  stem  with  the  growth  of  every  new  set  of 
leaves,  the  medullary  rays  ultimately  become  so  much  com- 
pressed, that  they  assume  the  form  of  fine  lines  radiating  from 
the  centre  to  the  circumference  of  the  stem. 

5* 


50  COMPOUND   ORGANS   OF  PLANTS. 

In  exogenous  stems  of  a  single  year's  growth  we  therefore 
observe,  a  central  cellular  pith,  a  zone  of  woody  fibre  and  vas- 
cular tissue,  an  exterior  coating  of  bark,  and  medullary  rays 
passing  from  the  pith  to  the  bark.  This  is  the  complete  struc- 
ture of  exogenous  herbaceous  stems  which  die  down  to  the 
ground  annually. 

In  exogenous  stems  which  are  not  annual,  at  the  close  Of 
the  growing  season  the  stem  ceases  to  elongate,  the  old  leaves 
gradually  fall  off,  the  new  leaves,  instead  of  expanding  after 
their  formation,  retain  their  rudimentary  condition,  harden  and 
fold  over  one  another,  and  a  bud  is  produced,  the  winter's 
residence  of  the  shoot. 

The  second  year's  growth. — The  next  year,  with  the  return  of 
light  and  heat  to  the  earth  in  Spring,  vegetation  re-commences. 
The  resinous  exudation  on  the  buds  is  melted  by  the  heat  of 
the  sun ;  the  scales  fall  off,  the  leaves  expand,  and  are  sepa- 
rated by  the  growth  of  the  internodes,  the  buds  terminal  and 
lateral  are  in  this  manner  elongated  into  shoots,  and  are  now 
to  the  parent  shoot  what'  the  young  leaves  were  to  it  the  first 
year ;  that  is,  they  perform  precisely  the  same  functions,  and 
contribute  by  their  downward  growth,  and  their  deposit  of 
woody  and  fibrous  matter,  to  the  increased  diameter  of  the 
parent  shoot. 

With  the  development  of  the  buds  into  shoots  and  leaves, 
the  sap  is  set  into  circulation  through  the  system  of  the  plant, 
and  the  bark  and  wood  which,  at  the  close  of  the  growing 
season,  or  in  autumn,  firmly  adhered  together,  are  now  easily 
separable  from  each  other,  by  the  formation  between  them  of 
a  stratum  of  mucilaginous,  organizable  matter,  termed  cam- 
bium. This  cambium  is  nothing  more  than  the  ordinary  sap, 
the  water  of  which  having  been  evaporated  in  the  leaves,  is 


ORGANIZATION    OF   THE   STEM.  51 

necessarily  thickened  and  well  charged  with  assimilated  mat- 
ters, and  is  interposed  between  the  wood  and  bark  where 
growth  is  going  on.  "  It  is  quite  wrong/'  says  Dr.  Gray,  "  to 
suppose  that  there  is  any  real  interruption  between  the  wood 
and  the  bark  at  this  or  any  other  period  of  time,  leaving  a 
space  filled  with  extravasated  sap.  A  series  of  delicate  slices 
will  at  any  time  show  that  the  bark  and  wood  are  always 
organically  connected  -with  each  other,  by  a  very  delicate  tissue 
of  vitally  active  partly  grown  cells."  The  cambium  thus 
deposited  between  the  wood  and  bark  becomes  organized  into 
cells,  and  forms  a  new  addition  of  matter  to  each.  Hence,  the 
forming  stratum  is  termed  the  cambium  layer,  the  inner  por- 
tion of  which  forms  wood,  and  the  outer,  bark.  It  is  when 
this  process  of  growth  is  most  rapidly  going  on,  in  spring  or 
early  summer,  and  the  whole  cambium  layer  is  gorged  with 
the  flow  of  sap,  that  the  bark  and  wood  are  so  easily  separated. 
But  the  separation  is  effected  by  the  rending  of  a  delicate  new 
tissue. 

At  the  end  of  the  second  year,  the  cambium  layer  of  new 
wood  and  bark  hardens,  the  second  annual  layer  or  ring  of 
wood  and  bark  is  formed,  and  the  bark  and  wood  again  adhere 
firmly  together.  The  new  shoots  are  prepared  for  winter  in 
precisely  the  same  manner  as  the  first  year's  shoot  was  pre- 
pared, and  are  elongated  cones  as  was  the  first. 

In  like  manner  will  the  plant  continue  to  grow  throughout 
the  third,  fourth,  and  succeeding  years,  each  annual  growth 
being  only  a  repetition  of  the  same  phenomena. 

After  a  certain  number  of  years  the  tree  arrives  at  the  full 
perfection  of  its  growth,  the  outer  layers  of  bark  now  become 
fissured  and  rent,  and  are  exfoliated  or  thrown  off  from  the 
stem,  and  the  alburnum  or  sapwood  becomes  changed  into 


52  COMPOUND   ORGANS    OF   PLANTS. 

duramen  or  heartwood,  which  ultimately  decays  and  falls  away 
leaving  the  interior  of  the  stem  rotten  and  hollow.  These 
changes  in  the  external  and  internal  appearance  of  the  stem 
are  the  necessary  results  of  the  following  peculiarities  of  its 
growth. 

We  have  seen  that  one  layer  of  bark  and  one  layer  of  wood 
is  annually  deposited  from  the  viscid  mucilaginous  matter 
called  cambium,  which  makes  its  appearance  between  the  bark 
and  the  wood  in  spring.  It  follows,  that  the  number  of  annual 
layers  or  rings  of  bark  ought  to  correspond  to  the  number  of 
annual  layers  or  rings  of  wood.  Sometimes  in  the  bark  of 
young  shoots  of  two  or  three  years  growth  these  annual 
deposits  may  be  traced,  but  in  general  the  successive  layers  of 
bark  are  so  amalgamated  by  the  internal  growth  and  conse- 
quent pressure  of  new  strata  of  bark,  that  it  is  impossible  to 
distinguish  them.  To  the  same  cause  is  to  be  attributed  the 
fissuring  and  exfoliation  of  the  outer  layers  of  bark.  The 
diameter  of  the  wood  is  a  constantly  increasing  quantity, 
because  the  growth  of  the  wood  is  exogenous,  each  new  layer 
of  wood  being  deposited  on  the  outside  of  the  last  annual  layer, 
and  therefore  each  ring  of  the  wood  remains  unaltered  in  its 
dimensions  and  position  until  it  finally  decays ;  on  the  other 
hand,  an  increase  in  the  diameter  of  the  bark  is  constantly 
prevented  by  the  endogenous  growth  of  the  bark,  each  new 
layer  of  bark  being  deposited  on  the  inside  of  the  last  annual 
layer ;  and  as  new  layers  of  bark  are  deposited  internally,  the 
previous  annual  layers  are  subjected  to  gradual  but  incessant 
distention,  and  finally  unable  to  bear  the  stretch,  are  fissured 
and  torn  into  clefts  and  rents,  causing  that  cracked  and  rugged 
appearance  of  the  bark  of  trees  with  which  all  are  familiar. 
Hence  it  is  that  on  the  cross-section  the  bark  bears  but  a 


ORGANIZATION   OP   THE    STEM.  53 

small  proportion  in  thickness  to  the  wood;  the  amount  of  bark 
which  remains  deposited  about  the  wood  is  exactly  propor- 
tionate to  the  stretch  or  tension  to  which  it  will  submit,  vary- 
ing greatly  in  different  species.  In  the  old  trunks  of  some 
pines  and  firs,  it  sometimes  attains  the  thickness  of  from  eight 
to  twelve  inches,  whilst  in  the  Platanus  occidentalis,  or  com- 
mon plane-tree,  after  the  eighth  or  tenth  year,  all  the  epi- 
phlceum  or  old  and  outer  layers  of  bark  fall  away  entirely  in 
the  form  of  brittle  plates. 

The  duramen  or  heartwood. — The  sap  chiefly  circulates  in 
the  inner  bark  and  alburnum  where  growth  is  going  on,  the 
new  and  fresh  tissues  being  most  active  in  its  transmission. 
The  walls  of  the  cells  soon  begin  to  thicken  by  the  internal 
deposition  of  mineral  matter  or  sclerogen  imbibed  through 
the  pores  of  the  roots  with  the  sap,  and  what  was  once  sap- 
wood  is  every  year,  by  the  development  of  new  rings  of  wood 
removed  farther  and  farther  from  the  region  of  growth ;  after 
a  few  years,  therefore,  it  ceases  to  take  part  in  the  vital  opera- 
tions of  the  plant,  its  color  changes,  and  it  becomes  what  is 
called  duramen  or  heartwood 

As  the  duramen  or  heartwood  doe's  not  assist  in  maintaining 
the  functions  of  the  tree,  it  may  decay  without  injury  to  the 
vitality  of  the  plant.  Hence  it  is  that  we  sometimes  see  old 
trees  covered  with  the  most  •  luxuriant  foliage,  although  their 
inside  is  totally  gone. 

Having  taken  a  cursory  view  of  the  development  of  an 
exogenous  stem,  from  the  period  when  it  first  emerges  from  its 
cotyledons  or  seed  leaves,  to  that  term  of  ifs  existence  when  it 
begins  to  show  signs  of  decay  in  its  interior,  we  shall  now 
attempt  a  more  careful  analysis  of  the  different  layers  of  bark, 


54  COMPOUND  ORGANS  OF  PLANTS. 

wood,  and   pith  which   the   stem  exhibits   on   its  transverse 
section. 

Fig.  11. 


Fig.  11  shows  a  transverse  section  A,  and  vertical  section  B,  of  an  exogenous  or 
dicotyledonous  stem  of  three  years'  growth.  "  In  both  sections,  a  represents  the 
cellular  tissue  of  the  pith,  6  &,  the  dotted  ducts,  and  c  c,  the  woody  fibre  of  the 
successive  annual  layers;  d  d,  the  spiral  vessels  of  the  medullary  sheath ;  e  e} 
cambium  layer ;  ff,  liber ;  g  g,  cellular  envelope ;  h  h,  corky  layer  ;  i  i,  medullary 
rays.  In  the  vertical  section  the  medullary  ray  is  shown  in  only  part  of  its 
length ;  since  the  continuity  of  the  medullary  rays  from  the  pith  to  the  bark,  owing 
to  the  slight  flexure  which  always  occurs  in  them,  is  rarely  or  never  shown  by  such 
a  section."* 

The  Bark. — From  this  section,  it  is  evident  that  the  bark, 
anatomically  considered,  may  be  subdivided  into  four  parts. 

1.  'The  epidermis  or  general  outer  integument.  The  nature 
of  this  investment  has  been  already  examined  pp.  (18-24).  It 
only  remains  at  preaent  to  add,  that  in  forest  trees  and  larger 


*  See  Carpenter's 
3d  edition,  1851. 


'Principles  of  Physiology,  General  and  Comparative," 


ORGANIZATION  OP  THE   STEM.  55 

shrubs,  the  bodies  of  which  are  of  a  firm  and  vigorous  texture, 
it  is  a  part  of  little  importance,  excepting  in  the  young  and 
tender  state  of  the  plant;  but  in  reeds,  grasses,  and  other 
plants  with  hollow  stems,  it  is  of  great  use  and  is  exceedingly 
strong,  being  chiefly  composed  of  silica,  or  flint.  The  epidermis 
is  not  represented  in  this  section. 

2.  The  epiphlceum  (exi  upon,  $71010$  bark),  or  corky  envelope, 
shown  in  the  section  at  h  h.     This  is  the  outer  covering  of  the 
bark,  and  consists  of  cubical  or  flattened  tubular  cells,  without 
chlorophyl,  placed  close  together  and  elongated  in  a  horizontal 
direction.     It  is  this  part  of  the  bark  which  gives  to  the  trunks 
of  trees  their  peculiar  color  and  rugged  appearance ;  generally 
some  shade  of  ash-color  or  brown. 

In  Quercus  suber,  the  cork  oak,  the  epiphkeum  consists  of 
numerous  strata  of  cells,  forming  the  substance  called  cork ; 
hence  the  name  corky  envelope,  which  is  given  to  it.  So  also 
the  branches  and  branchlets  of  Liquidambar  styraciflua,  the 
sweet  gum-tree,  and  of  Ulmus  racemosa,  one  of  the  elms  of  the 
northern  United  States,  are  winged  with  corky  ridges,  the 
result  of  an  unusual  development  of  the  epiphlceuni  of  the  bark. 
In  the  currant  and  birch,  the  epiphloeum  is  composed  of  only 
a  few  layers  of  cells,  and  may  be  seen  peeling  off  in  thin  cir- 
cular pieces  from  the  trunks  of  these  trees.  When  the 
epiphloaum  is  very  thick,  it  is  simply  fissured  or  rent,  in  which 
state  it  remains  attached  to  the  outside  of  the  stem,  forming  an 
excellent  protective  envelope  to  the  inner  and  vitally  active 
layers  of  bark. 

3.  The  mesophlceum  or  cellular  envelope,  represented  at  g.  g. 
This  lies  immediately  on  the  outside  of  the  liber.     Its  cells 
contain  chlorophyl,  and  are  developed  vertically.     It  is,  there- 
fore, that  part  of  the  bark  which  is  colored  green,  and  which 


56  COMPOUND   ORGANS   OF  PLANTS. 

gives  to  young  shoots  their  green  hues,  as  it  shines  through  the 
transparent  membrane,  the  cuticle.  In  the  shoots  of  young 
trees,  in  the  spring,  it  may  be  readily  perceived.  The  bark  of 
a  young  stem  the  first  year  always  assumes  this  color,  from 
the  production  of  chlorophyl  in  its  superficial  cells,  owing  to 
their  direct  exposure  to  the  solar  light. 

The  mesophloeum  or  green  layer  of  bark,  does  not  grow  at 
all  after  the  first  or  second  year.  It  is  excluded  from  the  light 
by  the  gradually  exterior  deposition  of  layers  of  epiphlceum, 
and  finally  perishes  never  to  be  renewed  again. 

4.  The  endophlmum  or  inner  bark,  called  also  the  liber,  ff. 
This  constitutes  the  fibrous  portion  of  the  bark,  the  corky  and 
cellular  envelopes  being  composed  exclusively  of  cellular  tissue. 
It  is  in  the  fibrous  portion  of  the  bark  that  the  sap  vessels  are 
contained,  which  convey  the  sap  from  the  roots  to  the  highest 
extremities  of  the  plant ;  hence  the  endophlceum  continues  to 
grow  throughout  the  life  of  the  plant,  being  formed  in  conjunc- 
tion with  the  alburnum  or  sapwood  directly  from  the  cambium 
layer. 

The  endophloeum  or  inner  bark  possesses  considerable 
strength  and  many  useful  properties.  The  inner  layers  of 
Tilia  Europoea,  or  the  lime  tree,  when  separated  by  maceration 
in  water,  form  the  common  bass  or  matting  used  by  gardeners, 
and  the  woody  fibre  which  is  used  for  the  manufacture  of 
cordage  in  all  exogenous  plants,  as  in  hemp,  flax,  &c.,  belongs 
to  the  endophloeum  or  inner  bark,  and  not  to  the  wood. 

The  cambium  layer,  e  e.  This  layer  has  been  already 
described,  pp.  (50-1).  In  herbaceous  plants  it  is  not  able  to 
organize  itself,  because  the  stem  dies  down  to  the  ground  the 
first  year.  In  every  other  respect  the  herbaceous  stem  offers 
the  same  structure  as  the  ligneous,  being  composed  equally  of 


ORGANIZATION   OP   THE    STEM.  57 

bark,  of  wood,  and  of  pith,  but  the  cambium  layer  is  not  there, 
and  therefore  it  wants  the  elements  necessary  to  the  forma- 
tion of  a  new  layer  of  bark  and  a  new  layer  of  wood. 

In  the  second  year,  the  gelatinous  tissue  of  the  cambium  is 
subjected  to  the  following  changes.  We  have  seen  that  it  occu- 
pies an  intermediate  position  between  the  bark  and  the  wood. 
This  zone  of  cambium  cells  produces  at  every  point  beds  of  the 
same  nature  as  those  with  which  it  is  in  immediate  contact, 
and  is  developed  into  ligneous  and  cortical  fibre,  preserving  its 
cellular  organization  only  in  those  portions  which  correspond 
to  the  medullary  rays.  The  inner  portion  of  the  cambium 
layer  forms  the  wood,  the  outer  portion  the  bark,  and  the  new 
cells  of  both  layers  thus  mould  themselves  entirely  on  the 
older  cells  throughout  all  their  points  of  contact. 

That  the  bark  increases  in  diameter  by  the  deposition  of  new 
layers  of  bark  internally,  was  first  proved  by  Duhamel,  a  cele- 
brated French  physiologist,  by  the  following  simple  experiment. 
He  passed  a  metallic  thread  between  the  liber  and  wood  of  a 
young  tree,  and  cutting  down  the  tree  several  years  after,  he 
found,  on  examination  of  the  cross-section,  that  rings  of  bark 
coinciding  in  number  with  the  years  elapsed  since  he  placed 
the  wire  next  the  wood,  had  grown  between  the  wire  and  the 
wood,  so  that  the  wire  was  separated  from  the  wood  by  a  con- 
siderable thickness  of  bark.  No  experiment  can  be  imagined 
more  decisive  than  this,  of  the  growth  of  the  bark  in  diameter 
by  internal  deposition  of  matter. 

The  wood. — This  consists  of  two  parts,  the  alburnum  or  sap 
wood,  and  the  duramen  or  heart  wood.  The  alburnum  or  sop 
woody  so  called  because  the  sap  circulates  through  it,  and  also 
in  allusion  to  its  white  or  pale  color.  The  alburnum  is  the 
zone  or  ring  of  wood  last  formed.  It  consists  of  elongated 

6 


58  COMPOUND  ORGANS  OF  PLANTS. 

tubes  of  woody  fibre,  c  c,  intermixed  with  bothrenchyma  or 
porous  vessels,  6  6.  On  holding  up  a  thin  traverse  section  of 
an  oak  or  ash  stem  to  the  light,  the  porous  vessels  will  be  seen 
in  the  form  of  large  round  openings  in  the  tissue,  which, 
situated  near  the  margin  of  each  woody  circle  or  zone,  renders 
apparent  the  annual  growths  of  the  stem.  In  the  maple, 
plane,  and  lime  tree,  these  openings  are  smaller  and  more  dif- 
fused, and  hence  there  is  an  indistinctness  in  the  line  of  demar- 
cation between  the  successive  zones. 

This  new  layer  of  wood  gradually  loses  its  softness  as  the 
season  advances,  and  towards  the  middle  of  winter  is  condensed 
into  a  solid  ring  of  wood.  In  this  country,  and  in  Europe 
generally,  there  is  a  periodical  check  to  vegetation  during  the 
colder  part  of  the  year,  which  occasions  the  annual  layers 
found  in  the  stem  of  exogenous  trees.  These  annual  rings, 
which  are  distinctly  seen  in  most  trees  of  temperate  climates 
when  a  section  of  their  stem  is  examined,  serve  as  natural 
marks  by  which  to  distinguish  their  age.  Thus,  suppose  an 
elm,  or  any  other  tree  to  be  felled,  and  the  section  near  the 
ground  to  have  thirty-five  circles,  or  rings  of  wood,  it  may  be 
inferred  that  the  tree  is  thirty-five  years  of  age. 

This  computation,  however,  can  only  be  made  in  trees  which 
have  these  rings  distinctly  marked,  and  even  then  there  are 
sources  of  deception  of  which  it  is  proper  that  the  student 
should  be  informed.  For  example,  a  warm  spring  followed  by 
weather  cold  enough  to  check  vegetation,  will  leave  a  ring  in 
the  stem,  and  the  subsequent  growth  of  the  stem/with  the 
return  of  warm  weather  will  give  on  the  cross-section  the 
appearance  of  two  rings  of  wood,  or  of  two  years  growth,  to  the 
growth  of  one  year ;  on  the  other  hand  a  warm  winter,  by 
keeping  the  tree  constantly  growing  without  check,  will  give 


ORGANIZATION    OF   THE    STEM.  59 

the  appearance  of  one  layer  to  the  growth  of  two  years.  Not- 
withstanding this,  practical  men  find  counting  the  concentric 
circles  of  exogenous  stems,  to  be  the  best  mode  which  has  yet 
been  discovered  for  ascertaining  their  age,  as  in  ordinary  cases 
only  one  growth  is  made  in  the  course  of  a  year. 

In  tropical  countries,  where  the  temperature  is  compara- 
tively speaking  pretty  much  the  same  throughout  the  ,year, 
these  rings  are  very  indistinctly  marked.  In  tropical  coun- 
tries, vegetation  is  not  liable  to  that  periodical  check  which  it 
receives  in  colder  regions,  and  therefore  the  cross-section  of 
the  stems  of  exogenous  trees  in  many  instances  do  not  disclose 
these  rings,  or  any  separation  of  the  wood  into  concentric 
layers. 

As  the  same  development  of  woody  and  cortical  matter  takes 
place  in  the  branches  as  well  as  in  the  stems  of  exogenous 
trees,  therefore,  the  time  when  a  branch  was  first  given  off 
from  the  stem  may  be  computed  by  counting  the  circles  on  the 
stem  and  branch  respectively.  If  there  are,  for  instance, 
thirty  rings  in  the  stem,  twenty  in  one  of  its  branches,  and 
five  visible  on  the  cross-section  of  another,  then  the  tree  must 
have  been  ten  years  old  when  the  first  branch  was  developed, 
and  twenty-five  years  of  age  when  it  formed  the  second. 

If  we  carefully  examine  the.  rings  on  the  cross-section  of  an 
exogenous  stem,  we  shall  soon  perceive  that  they  have  not  the 
same  geometrical  centre,  that  their  breadth  varies,  and  that 
they  are  occasionally  thicker  on  one  side  of  the  tree  than  on 
the  other.  A  variety  of  causes  contribute  to  produce  these 
effects.  The  variable  breadth  of  the  rings  depends  on  the 
variability  of  the  seasons ;  for  more  wood  will  necessarily  be 
deposited  when  the  season  is  favorable  for  vegetable  growth, 
than  when  the  contrary  is  the  case.  Moreover  the  circles  will 


60  COMPOUND    ORGANS    OF   PLANTS. 

be  broadest  on  the  side  of  the  tree  where  there  is  the  most 
wood  deposited,  and  this  is  invariably  on  the  side  which 
contains  the  greatest  number  of  branches  and  leaves.  In  a 
solitary  tree,  other  conditions  being  favorable,  the  rings  are 
generally  the  broadest  on  thje  south  side  of  the  stem. 

The  Duramen  or  Heart-wood. — We  have  already  stated 
that  the  walls  of  the  fully  developed  fibre-cells  through  which 
the  sap  circulates,  become  thickened  by  the  deposition  of 
matter  in  layers  on  their  interior  surface,  until  at  length  the 
cavities  of  the  cells  are  almost  entirely  closed;  when  this 
happens,  the  sap  can  no  longer  permeate  the  walls  of  the  cells, 
and  their  vital  functions  cease. 

This  solidification  of  the  wood-cells  is  usually  connected 
with  a  change  in  the  color  of  the  wood,  more  or  less  marked. 
Sometimes  this  change  is  made  suddenly  and  without  any 
intermediate  shades,  as  in  the  wood  of  the  ebony  and  logwood, 
of  which  the  heart-wood  is  black  or  deep  red  and  the  albur- 
num almost  white ;  but  it  frequently  happens,  that  there  exists 
no  sensible  difference  of  color  between  the  alburnum  and  heart- 
wood,  as  for  example,  in  trees  with  white  wood,  the  color  of 
the  heart-wood  never  changing  except  from  incipient  decay. 

This  older,  more  solidified,  and  harder  wood,  which  occupies 
the  centre  of  the  trunk,  is  the  part  principally  valued  by  work- 
men as  most  suitable  for  economic  purposes.  The  various  fancy 
colored  woods  employed  by  the  turner  and  cabinetmaker  con- 
sist of  the  heart-wood  only,  which  assumes  different  colors  in 
different  species,  being  black  in  the  ebony,  bright  yellow  in  the 
barberry,  purplish-red  in  the  cedar  wood,  and  dark-brown  in  the 
black  walnut.  The  alburnum  in  all  these  trees,  even  in  the 
ebony  itself,  is  always  white,  and  is  regarded  by  workmen  as 
a  part  of  the  tree  of  little  or  no  value. 


ORGANIZATION   OF   THE    STEM.  61 

The  ligneous  zones,  considered  collectively,  are  more  indu- 
rated towards  the  interior  of  the  stem,  because,  in  fact,  these 
are  the  most  ancient  deposits ;  but,  examined  separately,  they 
are  more  solid  in  their  exterior  than  in  their  interior  parts; 
because  the  latter  was  formed  in  early  spring,  at  a  period  when 
the  nutritive  sap  was  more  aqueous  and  less  condensed. 

The  Medullary  Sheath. — This  is  a  very  thin  zone  of  vascu- 
lar tissue  and  spiral  fibre,  immediately  surrounding  the  pith, 
shown  at  dd.  It  may  be  readily  seen  in  the  traverse  section 
of  a  young  exogenous  shoot  by  its  green  color,  which  appears 
deeper  as  contrasted  with  the  white  of  the  pith  which  it  sur- 
rounds. If  we  scoop  out  the  pith  of  the  shoot  from  the 
ligneous  cylinder  which  surrounds  it,  we  shall  obtain  a  longi- 
tudinal view  of  the  medullary  sheath,  which  will  appear  like 
a  green  layer  on  the  interior  surface  of  the  cylinder.  The 
medullary  sheath  is  the  earliest  formed  portion  of  the  vascular 
system,  and  is  developed  with  the  upward  elongation  of  the 
stem,  sending  its  woody  fibre  and  spirals  into  each  young  shoot 
and  leaf  to  form  its  veins.  The  medullary  sheath  is  the  only 
part  of  an  exogenous  stem  in  which  spiral  vessels  occur. 

The  Pithy  represented  at  a,  consists  of  soft  cellular  tissue, 
and  is  formed  as  the  stem  elongates.  At  first  it  abounds  with 
nutritive  matter,  which  serves  to  nourish  the  growing  bud 
resting  on  its  summit ;  this  office  fulfilled,  it  becomes  dry  and 
dies,  assuming  the  appearance  and  structure  of  wood,  insomuch 
that  in  old  stems,  there  is  scarcely  such  a  thing  as  pith  to  be 
seen.  Herbs  and  young  shrubs,  in  proportion  to  their  bulk, 
have  more  pith  than  trees.  Many  herbaceous  stems  expand  so 
rapidly  during  their  early  growth  that  they  become  hollow, 
the  pith  being  torn  away  by  the  distension  and  its  remains 

6* 


62  COMPOUND  ORGANS  OF  PLANTS. 

forming  a  mere  lining  to  the  cavity,  as  in  grasses  and  other 
herbs. 

A  species  of  jEschynomene,  growing  in  China,  has  the  whole 
of  its  stem,  which  is  about  an  inch  thick,  composed  of  a  mass 
of  pith,  covered  by  a  very  thin  epidermis.  Rice-paper  is  pro- 
cured from  the  herbaceous  stem  of  this  plant  by  the  following 
process.  The  centre  column  of  pith  is  cut  spirally  round  the 
axis  with  a  sharp  instrument  into  a  thin  lamina,  which  is  then 
unrolled,  and  may  be  made  into  sheets  containing  about  a  foot 
square.  The  medullary  sheath  and  the  concentric  zones  of 
wood  are  traversed  by 

The  Medullary  Rays. — These  are  numerous  thin  plates  of 
condensed  cellular  tissue,  which  pass  from  the  pith  to  the  cellu- 
lar system  of  the  bark,  and  maintain  a  communication  between 
them.  These  plates  of  cellular  matter  are  to  be  seen  on 
the  surface  of  the  cross-section  of  most  exogenous  stems, 
on  which  they  appear  as  fine  lines,  radiating  from  the  centre 
to  the  circumference,  but  cannot  be  traced  continuously  to 
any  great  extent  in  a  vertical  direction.  The  medullary 
rays  constitute  the  silver  grain  of  the  carpenters.  They  are 
the  remains  of  the  cellular  system  of  the  stem,  condensed  into 
lines  by  the  adjacent  pressure  of  the  woody  wedges.  The  cellu- 
lar system  of  the  stem  is  first  formed  in  a  horizontal  direction, 
and  constitutes  the  matrix,  or  bed,  into  which  ascends  and 
descends,  in  a  longitudinal  direction,  the  fibro-vascular  or 
woody  system.  The  wood  of  the  exogen  is,  in  fact,  made  up 
of  a  number  of  wedges  of  longitudinal  fibro-vascular  tissue, 
embedded  in  the  horizontal  cellular  tissue  of  the  stem.  The 
base  of  each  wedge  is  in  contact  with  the  inner  surface  of  the 
bark;  the  apex  is  next  the  pith  and  its  sides  are  bounded  by 
the  medullary  rays,  which  are,  as  before  stated,  the  remains  of 


ORGANIZATION    OP   THE    STEM.  63 

the  horizontal  cellular  system,  condensed  by  pressure  of  the 
longitudinal  wedges  into  fine  lines  of  cellular  matter. 

On  the  whole,  the  organization  of  an  exogenous  stem,  con- 
sidered collectively,  presents  three  distinct  systems,  the  cortical, 
ligneous  and  medullary  system,  or  the  bark,  the  wood  and  the 
pith ;  and  from  the  mode  of  its  development,  it  is  evident  that 
the  wood  has  a  constant  tendency  to  solidify,  itself,  and  the 
bark  to  destroy  itself;  hence  vitality  soon  ceases  in  the  former, 
and  the  exfoliation  and  fall  of  the  different  parts  of  the  latter, 
first  the  epidermis,  then  the  suberous  cellules,  the  cortical  pith 
and  even  the  liber. 

The  stem  of  an  old  exogenous  tree,  therefore,  consists  of  a 
curious  conjunction  of  dead  and  living  matter,  and  the  rings  of 
wood  not  only  mark  the  growths  of  successive  years,  but  the 
number  of  generations  of  spontaneously  grafted  individuals 
which  the  stem  has  sustained.  No  part  of  such  a  tree  is  alive 
now  that  was  living  a  few  years  ago.  The  leaves  have  fallen 
which  the  tree  then  bore,  and  the  nodes  from  which  they 
sprung  are  deeply  buried  in  the  interior  of  the  stem,  beneath 
the  wood  formed  by  the  generations  of  buds  and  leaves  that 
succeeded  them;  whilst  the  living  bark  that  then  covered  the 
stem  in  immediate  contact  with  the  wood  has  been  separated 
from  it  by  the  internal  growth  and  deposition  of  other  strata 
of  bark,  and  is  now  visible  on  the  outside  of  the  stem  in  the 
form  of  dead  and  fissured  layers,  or  else  has  been  thrown  off 
from  its  surface  altogether.  Thus  in  the  coral  tree,  far  beneath 
the  ocean  wave,  where  mineral  matter  assumes  a  vegetable 
form,  the  recent  shoots  and  surface  alone  are  alive,  all  is  dead 
along  the  central  axis. 


64  COMPOUND   ORGANS   OF   PLANTS. 

|HI*   • 

CHAPTER   V. 

ON  THE  DEVELOPMENT  OF  THE  BUDS  AND  BRANCHES. 

THE  stem  or  aerial  portion  of  the  axophyte  possesses  exclu- 
sively a  force  of  lateral  expansion,  by  means  of  which  it 
projects  into  the  atmosphere  numerous  dilated  appendages,  in 
the  form  of  membranaceous  expansions  of  its  cells  and  fibres, 
more  or  less  flattened,  and  of  a  green  color,  which  are  termed 
leaves.  Certain  definite  cells  of  the  axophyte  appear  to  have 
a  natural  tendency  to  this  lateral  growth,  and,  therefore,  these 
leaves  are  produced  symmetrically  at  certain  definite  points  on 
the  stem  called  nodes  (nodus,  a  knot;)  so  called  because  these 
parts  of  the  stem  are  internally  more  solid  and  compact  than 
the  other  parts,  in  consequence  of  the  vertical  fibres  of  the 
stem  being  interwoven  with  those  which  are  sent  off  horizon- 
tally into  the  leaf.  These  nodes  are  very  conspicuous  in  the 
bamboo,  Indian  corn,  and  all  plants  with  hollow  stems,  which 
stems,  on  examination,  will  be  found  to  be  solid  at  these 
points.  The  naked  intervals  of  stem  between  the  nodes  are 
termed  internodes. 

Before  their  expansion  these  leaves,  together  with  the 
branches  on  which  they  are  borne,  are  enclosed  in  a  particular 
organ  termed  a  bud.  All  branches  begin  and  terminate  in  a 
bud.  .A  bud  is,  therefore,  clearly  an  undeveloped  branch. 

Now  the  bud,  or  undeveloped  branch  or  stem,  is  made  up  of 
a  succession  of  these  leaf-bearing  points  or  nodes,  the  inter- 
nodes  between  which  have  not  been  developed,  so  that  these 
nodes  or  leaf-bearing  points  are  brought  into  close  proximity, 


BUDS   AND    BRANCHES. 
Fig.  12. 


65 


A  year's  growth  of  the  horse-chestnut  branch,  crowned  with  a  terminal  bud;  a, 
scars  left  by  the  bud-scales  of  the  previous  year;  b,  leaf-scars,  with  round  dots,  show- 
ing the  points  of  issue  of  the  fasciculi,  or  bundles  of  woody  fibre  which  form  the 
petiole;  c,  axillary  buds,  developed  at  the  base  of  the  petiole  of  the  fallen  leaves. 

and  the  leaves  themselves  developed  in  a  rudimentary  state, 
assume  a  scale-like  appearance,  and  overlap  each  other  symme- 
trically in  accordance  with  their  natural  arrangement  on  the 
stem.  The  formation  of  buds  is  the  natural  result  of  the 
cessation  of  the  growth  of  the  internodes,  and  the  partial 
development  of  the  leaves  at  the  nodes. 

That  the  scales  of  buds  are  leaves  in  an  imperfectly  formed 
or  rudimentary  state,  is  evident  from  the  fact  that  they  are  the 


66  COMPOUND  ORGANS  OF  PLANTS. 

last  leaves  of  the  season,  developed  at  a  period  when  the  sap 
is  ceasing  to  flow,  and  when  the  vital  powers  of  plants  have 
become  almost  torpid.  The  transition  of  scales  into  the 
ordinary  leaves  of  the  stem  is  well  seen  in  the  spring,  in  the 
expanding  buds  of  the  hickory  or  horse-chestnut,  where  the 
gradual  passage  of  one  into  the  other  may  be  distinctly 
traced. 

Buds  originate  in  the  horizontal  or  cellular  system,  and  may 
be  distinctly  traced  in  young  branches  to  the  pith  or  medullary 
rays,  at  the  extremities  of  which  they  are  invariably  found 
when  they  take  a  lateral  development.  This  may  be  easily 
verified  by  making  a  section  through  the  centre  of  one  of  the 
lateral  buds,  at  right  angles  to  the  surface  of  the  stem,  when 
the  medullary  ray  will  be  seen  on  the  surface  of  the  section  in 
the  form  of  a  white  line,  which,  proceeding  from  the  centre  of 
the  bud,  traverses  the  several  rings  or  annual  deposits  of  wood, 
and  terminates  in  the  pith  at  the  centre  of  the  stem.  The 
central  cellular  portion  of  every  bud  is  therefore  in  direct 
communication  with  the  interior  pith  of  the  young  shoot  by 
means  of  the  medullary  rays,  at  the  extremities  of  which  they 
are  formed. 

Buds  are  formed — some  in  the  early  part  of  the  summer, 
others  late  in  autumn,  before  the  leaves  fall  from  the  trees — in 
the  axilla  of  the  leaves,  that  is,  in  the  angle  formed  by  the  leaf- 
stalk and  the  stem.  Examine  the  branch  of  any  tree  before  it 
has  cast  its  leaves,  and  you  will  find  at  the  base  of  the  petiole 
or  leaf-stalk,  the  buds  for  the  ensuing  year.  Hence  in 
autumn,  after  the  leaves  have  fallen,  these  buds  remain 
attached  to  the  branches. 

Linnaeus  called  buds  the  hybernaculum  or  winter  residence 
of  the  branch ;  and  the  term  is  very  appropriate,  because  it 


BUDS    AND    BRANCHES.  67 

expresses  admirably  the  purposes  for  which  the  buds  are 
formed. 

The  scales  which  envelope  the  bud  are  clearly  designed  to 
protect  the  embryo  branch  and  leaves  of  the  next  season,  which 
they  surround,  against  the  humidity  and  cold  of  winter.  They 
vary  in  their  texture,  external  covering,  and  thickness,  in  dif- 
ferent plants.  In  the  beech  and  lime  tree,  the  bud  scales  are 
thin  and  dry ;  in  the  willow  and  magnolia,  thick  and  downy, 
and  in  the  horse-chestnut  and  balsam  poplar,  they  are  covered 
externally  with  a  plentiful  exudation  of  gummy  resin,  and 
thickly  clothed  internally  with  a  woolly  substance.  By  this 
beautiful  provision  both  wet  and  cold  are  effectually  excluded. 
Plants  are  most  unquestionably  a  peculiar  form  of  life,  and 
when  we  see  them  thus  modifying  their  organs  to  escape  what 
is  hurtful  to  their  existence  in  the  air,  and  constantly  availing 
themselves  in  the  development  of  their  roots  of  what  is  con- 
ducive to  their  growth  in  the  earth,  we  must  admit  them  to  be 
somewhat  elevated  in  the  scale  of  nature  and  very  far  removed 
from  the  conditions  of  inorganic  matter. 

la  the  Smilax  rotundifolia,  or  common  green  brier,  the 
buds  are  protected  through  the  winter  by  the  dilated  and  per- 
sistent base  of  the  petiole  or  stalk  of  the  old  leaves,  which 
remains  on  this  shrub  throughout  the  winter  and  falls  away  in 
the  spring.  In  the  Platanus  occidentalis,  or  Plane  tree,  we 
seek  in  vain  for  the  buds  in  their  ordinary  situation,  the  axils 
of  the  leaves,  for  they  are  protected  during  their  growth,  and 
are  concealed  within  the  swollen  base  of  the  petiole.  This  is 
well  seen  in  autumn,  when,  on  removing  the  leaf  from  the 
stem,  the  base  of  the  stalk  is  found  to  form  a  cap  or  covering 
to  the  leaf  buds. 

Buds  contain  in  their  interior,  in  an  embryo  state,  the  whole 


68  COMPOUND   ORGANS   OP   PLANTS. 

plan  of  the  next  year's  growth,  the  nodes  and  even  the  leaves 
of  the  future  stem.  On  the  approach  of  winter  the  vegetable 
machinery  stops,  but  there  is  no  disarrangement  of  its  parts, 
on  the  contrary,  all  is  ready  in  the  bud,  and  awaiting  the 
stimulus  of  the  returning  light  and  heat. 

The  young  leaves  are  beautifully  folded  together  in  the  bud 
in  such  a  manner  as  to  occupy  the  least  possible  space,  the 
peculiar  mode  varying  in  different  plants.  The  arrangement  of 
the  leaves  in  the  bud  is  termed  their  vernation  or  prsefoliation. 
Any  one  can  examine  it  in  the  spring  with  the  certainty  of 
being  very  much  interested,  by  cutting  across  the  leaf-buds 
with  a  sharp  knife,  when  they  are  swelling  and  before  they 
have  begun  to  expand. 

On" the  approach  of  spring,  the  leaf  buds  throw  off  their 
scales,  and  the  leaves  which  were  at  first  all  crowded  and 
closely  packed  together  in  the  bud,  become  separated  from 
each  other  by  the  elongation  of  their  axis  of  growth,  or  the 
formation  of  internodes  or  naked  intervals  of  stem  between 
them,  much  after  the  mode  in  which  the  joints  of  a  pocket 
telescope  are  drawn  out  one  after  the  other;  whilst,  at  the 
base,  or  in  the  axilla  of  every  leaf-stalk,  is  seen  to  form,  as  the 
season  advances,  buds  capable  in  their  turn  of  being  developed 
into  branches,  or  a  provision  for  the  growth  of  the  ensuing 
season. 

Now  it  is  the  growth  of  the  terminal  bud  which  produces 
the  elongation  of  the  stem,  whilst  the  development  of  the 
axillary  buds  produces  the  branches ;  and  as  the  arrangement 
of  axillary  buds  depends  on  that  of  the  leaves,  in  the  axils  of 
which  they  grow,  and  as  the  bud  is  the  germ  of  the  future 
branch,  it  is  evident  that  the  development  of  the  branches, 
together  with  all  their  subsequent  ramifications,  must  follow 


BUDS   AND    BRANCHES.  69 

the  same  law  as  that  which  governs  the  arrangement  and 
position  of  the  leaves.  If  the  leaves  be  opposite,  the 
branches  will  be  opposite ;  if  the  leaves  be  alternate,  the 
branches  will  be  alternate ;  and  so  on.  This  symmetrical 
arrangement  of  the  branches  is  interfered  with  and  obscured  by 
the  operation  of  the  following  causes  : — 

The  non-development  of  some  of  the  axillary  buds. — As  the 
primary  plant  is  only  called  forth  from  seed  by  certain  condi- 
tions of  heat,  light  and  moisture  favorable  to  its  development, 
without  which  it  remains  latent  in  the  seed,  so  the  branches 
only  protrude  from  axillary  buds  when  circumstances  are 
favorable,  otherwise  the  buds  remain  latent  on  the  stem,  and 
no  branches  proceed  from  them.  Now  many  of  the  buds  in 
the  axils  of  the  leaves  do  not  grow ;  because  their  growth  is 
checked  by  the  rapid  growth  of  some  few  leading  buds,  which 
monopolize  all  the  nutriment,  leaving  them  only  just  sufficient 
to  carry  them  forward  with  the  increasing  thickness  of  the  stem, 
and  to  maintain  their  position  on  its  surface,  where  they  remain 
ready  for  action  in  case  the  growth  of  the  other  buds  is  checked 
by  untimely  frost,  or  other  causes.  In  this  manner,  trees, 
whose  young  and  tender  foliage  and  branches  has  sustained 
injury  by  the  cold  in  early  spring,  soon  become  re-clothed  with 
verdure.  On  this  principle,  also,  trees  are  pruned  and  trained 
against  walls,  or  other  supports.  Certain  leading  shoots  and 
buds  are  cut,  in  order  that  the  supply  of  sap  they  were  mono- 
polizing may  flow  to  certain  lateral  and  latent  buds,  and  cause 
their  growth  in  the  proper  direction.  In  general  the  sap  has 
tendency  to  rise  in  greater  force  and  abundance  towards  the 
extremity  of  the  branches,  the  result  of  this  is  that  the  inferior 
leaves  are  the  first  to  become  detached  from  the  branches,  and 
their  buds  not  receiving  enough  nourishment  to  bring  them  to 

7 


70  COMPOUND  ORGANS  OF  PLANTS. 

a  perfect  state,  become  abortive  or  incompletely  developed.  It 
is  in  fact  almost  always  the  inferior  buds  which  are  thus  reduced 
to  a  rudimentary  condition.  The  light  does  not  get  access  to 
them  so  freely  as  to  the  buds  towards  the  summit  of  the 
branches,  and  hence  the  lower  part  of  the  branches  is  generally 
naked  and  deprived  of  branchlets.  The  symmetrical  arrange- 
ment of  the  branches  with  the  leaves  is  also  prevented. 

By  the  growth  of  adventitious  or  irregular  buds,  that  is  to  say, 
of  buds  which  come  in  parts  of  the  stem,  between  the  leaves,  and 
not  in  their  place  in  the  axils  of  the  leaves.  Sometimes,  owing 
to  the  growth  of  the  leading  buds,  the  growth  of  the  latent 
axillary  buds  is  checked  altogether,  in  which  case  they  sink 
beneath  the  surface  of  the  stem,  and  are  buried  beneath  the 
succeeding  layers  of  wood ;  but  their  vitality  is  not  destroyed 
so  long  as  they  remain  at  a  certain  depth  in  the  stem,  that  is 
to  say,  in  the  alburnum  or  sap-wood.  The  trunks  and  branches 
of  trees,  therefore,  always  contain  an  immense  number  of  these 
buried  buds,  and  should  some  of  the  leading  branches  be  broken 
off  by  high  winds,  or  sustain  injuries  from  other  causes  of  this 
character,  then  the  flow  of  sap  to  them  becomes  so  powerful 
that  they  will  force  their  way  through  the  wood  to  the  surface, 
although  that  wood  be  the  successive  growths  of  years,  and 
break  forth  into  branches.  All  must  be  familiar  with  the 
sight  of  willows  and  other  trees,  whose  main  branches  have 
been  thus-  broken,  and  -whose  trunks  have,  nevertheless,  been 
covered  with  young  branches  and  shoots,  the  growth  of  buds 
which  "have  been  buried  in  their  wood,  and  for  years  dormant 
beneath  their  surface. 

From  these  facts  it  is  plain  that  those  forms  of  life  which  we 
call  plants,  although  rooted  to  the  soil,  and  more  exposed  by 
this  circumstance  than  any  other  living  being,  are  nevertheless 


BUDS   AND    BRANCHES.  71 

far  from  being  destitute  of  a  power  to  escape.  It  is  true  that 
they  are  exposed  to  the  inclemency  of  the  season,  and  are 
threatened  with  destruction  on  every  side,  but  so  powerful  and 
varied  are  the  defences  with  which  nature  has  furnished  them, 
that  they  seem  to  be  all  but  indestructible.  How  innumerable 
are  the  buds  with  which  a  tree  is  covered !  *  How  complete 
their  protective  apparatus  against  the  winter's  cold  !  We  have 
seen  that  each  bud,  although  it  remains  in  union  with  the 
parent  tree,  is  nevertheless  capable  of  .forming  the  germ  of  an 
independent  life.  If  not  developed,  it  only  awaits  the  destruc- 
tion of  its  associates  to  enter  the  breach,  repair  the  injury,  and 
continue  by  its  growth  the  battle  of  the  living  principle  in  the 
plant  against  the  hostile  forces  of  nature.  Endowed  with  such 
powers  of  defence,  a  tree  will  grow  and  lift  its  majestic  and 
massive  stem  for  centuries  to  the  air  and  light  of  heaven,  and 
if  after  thus  long  and  bravely  conflicting  with  nature,  it  should 
be  finally  prostrated  by  the  power  of  the  tempest,  if  its  con- 
nection with  the  soil  still  continues,  the  reserved  and  buried 
buds  of  other  years  shall  issue  forth  a  new  phalanx  of  defence, 
and  renew  successfully  the  struggle  of  the  plant  for  life. 


72  COMPOUND    OilGANS   OF   PLANTS. 

CHAPTER  VI. 

THE  LEAVES. 

LEAVES  are  contrivances  by  which  the  green  absorbent  sur- 
face of  the  plant  is  increased,  so  that  the  greatest  practicable 
amount  of  food  is  taken  from  the  air.  The  entire  structure  of 
the  leaf  proves  it  to  be  put  forth  for  this  purpose. 

The  leaf  is  simply  an  expansion  of  the  green  cellular  bark 
of  the  young  shoot,  and  is  formed  by  the  spread  of  the  woody 
fibre  which  issues  from  its  side,  carrying  with  it  at  the  same 
time  the  bark,  which  thus  becomes  expanded  horizontally  to 
the  air  and  light  of  heaven. 

When  the  leaf  is  fully  developed  it  consists  of  two  parts, 
viz.  :  the  expanded  portion  called  the  lamina  or  blade,  and  its 
little  stalk  or  support,  which  is  termed  the  petiole.  Some- 
times, however,  the  petiole  is  wholly  absent  from  the  leaf,  the 
spread  of  the  woody  fibre,  together  with  the  expansion  of  the 
green  young  bark  of  the  young  shoot,  taking  place  at  its  sur- 
face. In  this  case  the  leaf  is  said  to  be  sessile.  So  also  this 
expansion  of  bark  does  not  always  take  place  at  a  single  point 
of  the  stem,  but  is  extended  down  the  stem  a  little  and  then 
spreads  out  horizontally,  producing  a  decurrent  leaf.  The 
leaves  of  the  Verbascum  thapsus,  or  common  mullein,  are  of 
this  description.  Occasionally,  as  in  the  orange,  the  bark  of 
the  petiole  itself  shows  this  tendency  to  expansion,  when  the 
petiole  is  said  to  be  winged.  Most  frequently,  however,  dis- 
tinct fasciculi  or  bundles  of  woody  fibre  and  ^spiral  vessels 
emerge  from  the  side  of  the  shoot,  unite  and  form  a  petiole, 


THE   LEAVES.  73 

and  then  diverge  at  some  distance  from  the  stem,  forming  the 
expanded  lamina  of  the  leaf.  The  points  of  the  stem  from 
which  these  fasciculi  have  issued  are  apparent  on  the  scars  left 
by  the  fallen  leaf  stalks  in  the  form  of  round  dots,  of  a  uniform 
number  and  arrangement  in  each  species  of  plant.  Thus  in 
the  apple,  the  pear,  and  the  peach,  the  leaf  is  attached  to  the 
stem  by  three  fasciculi  or  bundles  of  woody  fibre,  and  three 
round  dots  may  be  distinctly  seen  on  the  leaf  scar ;  and  in  the 
horse-chestnut,  from  five  to  seven  dots  are  visible  on  the  leaf 
scar,  the  number  of  fasciculi  passing  out  of  the  stem  and 
uniting  in  the  petiole  being  the  same  as  the  number  of  the 
leaflets. 

The  vascular  or  woody  system  which  passes  out  of  the 
stem  into  the  leaf  is  clearly  designed  to  give  to  it  the  needful 
strength  and  support,  as  well  as  to  convey  the  sap  to  be  aerated 
in  the  leaf.  This  part  of  the  leaf  evidently  constitutes  its 
framework  or  skeleton.  The  vascular  and  woody  system  in 
exogenous  leaves,  as  for  example,  that  of  the  Cornus  florida,  or 
Flowering  dogwood,  consists  of  a  distinct  midrib  or  keel,  and 
less  elevated  ribs,  (costse,}  which  proceed  from  the  sides  of  the 
midrib  and  take  a  curvilinear  direction  to  the  margin  and 
apex  of  the  leaf.  On  closer  investigation  the  costee  are  seen 
to  communicate  with  each  other  by  means  of  small  transverse 
fibres,  which  again  branch  and  subdivide  in  various  ways,  the 
last  ramification  or  branchlets  running  together  or  anastomosing 
amongst  themselves,  and  the  whole  forming  a  delicate  and 
beautiful  network. 

Seen  through  the  microscope,  this  vascular  framework  is  found 
to  consist  of  woody  fibre  enclosing  spiral  vessels.  This  is  its 
constitution,  from  the  main  fasciculus  or  bundle  of  woody  fibre 
called  the  midrib  or  keel  of  the  leaf,  through  the  several  fasci- 


74 


COMPOUND   ORGANS   OP   PLANTS. 


culi  or  costae  which  proceed  from  the  sides  of  the  midrib,  each 
fasciculus  consisting  of  woody  fibre  enclosing  spiral  vessels 
throughout  all  its  ramifications. 

The  cellular  system  of  the  leaf. — This  substance  forms  its 
principal  part,  filling  up  the  meshes  in  the  network,  formed  by 
the  vascular  system.  To  the  naked  eye  it  appears  as  a  struc- 
tureless pulpy  mass  of  a  green  color,  called  parenchyma  (rfapa, 
beside  or  between,  and  gcv/ta,  anything  effused  or  spread  out.) 
Under  the  microscope  the  parenchyma  of  the  leaf  no  longer 
appears  as  an  unformed  mass,  but  as  a  beautiful  and  regular 
arrangement  of  cells,  which  are  so  disposed  as  most  effectively 
to  subserve  those  purposes  of  nutrition  for  which  the  leaf  is 
formed. 

In  all  leaves  which  present  one  surface  to  the  sky  and  the 
other  to  the  ground,  there  is  between  the  upper  and  under 
cuticle  two  strata  of  parenchyma  differently  arranged.  In  the 
upper  stratum  of  parenchyma,  the  cells  are  arranged  in  one  or 
more  compact  layers,  vertically,  or  at  right  angles  to  the  upper 
surface  of  the  leaf,  so  that  they  present  the  least  possible 

Fig.  13. 


Fig.  13.  Magnified  view  of  the  edge  of  a  leaf.  The  parenchyma  is  alone  represented, 
the  woody  tissue  being  left  out.  a  and  b,  show  the  epidermis  and  denser  parenchyma 
of  the  upper  surface  of  the  leaf;  c,  d,  the  looser  parenchyma  and  epidermis  of  its 
lower  surface. 


THE   LEAVES.  75 

amount  of  surface  to  the  sun ;  whilst,  in  the  lower  stratum  of 
parenchyma,  the  cells  are  arranged  horizontally,  having  amongst 
them  numerous  intercellular  passages,  or  cavities  filled  with  air, 
which  communicate  freely  with  each  other  through  the  sub- 
stance of  the  leaf,  and  with  the  external  air  by  means  of  the 
stomata  or  pores  in  the  epidermis. 

The  dense  parenchyma  of  the  upper  surface  of  the  leaf 
accounts  for  its  deeper  tint,  and  is  well  adapted  to  restrain 
the  excessive  evaporation  to  which  the  fluids  in  the  upper 
stratum  of  cells  are  liable,  by  their  direct  exposure  to  the  sun ; 
whilst  the  loose  parenchyma  of  the  lower  surface  is  the  cause 
of  the  lighter  tint  of  the  underside  of  the  leaf,  which,  together 
with  the  pores  of  the  cuticle,  is  well  calculated  to  give  the  air 
free  access  to  all  parts  of  the  leaf,  from  which  source  plants 
derive  the  greater  part  of  their  nutriment.  Leaves  growing 
erect  possess  uniformity  of  structure  in  both  strata  of  paren- 
chyma. 

The  vegetable  membrane  which  forms  the  walls  of  the 
cells  of  the  parenchyma,  is  perfectly  white  and  colorless. 
The  green  color  of  the  leaf  is  found  to  be  caused  by  the  forma- 
tion of  granules  of  green  matter  in  the  cells,  which  either  float 
free  in  the  sap  contained  in  their  cavities,  or  else  collect  into 
grains  and  adhere  to  the  walls  or  sides  of  the  cells.  This  sub- 
stance is  called  chlorophyl  (^wpo$,  green,  and  <J>VM.OV,  a  leaf),  in 
contradistinction  to  chromule  (^pc^ua,  a  color),  which  is  the 
term  employed  by  botanists  to  designate  the  colored  substances 
with  which  the  cells  of  flowers  are  filled. 

Chlorophyl  appears  to  belong  to  the  class  of  waxy  bodies. 
It  is  soluble  in  alcohol  or  ether,  but  not  in  water,  and  is 
developed  only  in  those  cells  which  are  exposed  to  the  action 
of  light.  It  is  therefore  only  formed  in  the  superficial  strata 


76  COMPOUND   ORGANS   OF   PLANTS. 

of  cells.  Chlorophyl  is  not  developed  in  the  external  investing 
layer  of  cells  called  the  epidermis,  nor  in  the  woody  fibre 
through  which  the  crude  sap  circulates,  both  the  epidermis 
and  woody  fibre  being  from  this  cause  white  and  transparent  j 
but  it  is  formed  in  the  superficial  strata  of  cells  immediately 
beneath  the  epidermis,  and  gives  to  the  leaves  and  young 
shoots  their  green  hues. 

That  the  chlorophyl,  or  green  matter  in  plants,  is  produced 
by  the  effect  of  light,  is  evident  from  the  fact  that  it  is  decom- 
posed and  disappears  when  plants  are  made  to  grow  in  the 
dark.  The  celery  served  at  table  is  blanched  or  rendered 
white  by  covering  the  stems  with  earth,  so  that  the  light 
cannot  gain  access  to  them ;  and  for  the  same  reason,  plants 
exposed  to  the  full  sunshine  have  a  deeper  tint  than  those 
which  grow  in  the  shade. 

The  epidermal  system  of  the  leaves,  together  with  the  val- 
vular action  of  their  pores  has  been  already  described,  (page 
22,)  and  we  have  seen  *how  beautifully  their  stomata  control 
the  evaporation  from  their  surface.  But  these  organs  have 
other  uses.  They  are  the  instruments  by  which  the  leaves 
communicate  directly  with  atmosphere,  and  by  which  vegetable 
breathing  or  respiration  is  carried  on.  Vegetables  respire  as 
well  as  animals,  and  the  sap  of  plants  which  is  analogous  to 
the  blood  of  animals,  must  be  brought  into  contact  with  the 
atmosphere,  like  the  blood,  and  be  thoroughly  aerated  in  the 
leaves,  before  it  can  be  converted  into  nutritive  fluid. 

Bonnet  was  the  first  who  observed  that  leaves,  when  plunged 
into  water  and  exposed  to  the  action  of  sunlight,  disengaged 
gas.  He  also  found  by  experiment,  that  the  same  amount  of 
gas  was  evolved  when  the  leaves  were  immersed  in  water 
which  had  been  previously  boiled,  and  therefore  completely 


THE    LEAVES.  77 

deprived  of  its  air.  The  gas  was  therefore  clearly  evolved  by 
the  leaves.  Priestly  recognized  this  gas  to  be  oxygen,  and 
Ingenhouse  showed  that  light  was  indispensable  to  its  manifes- 
tation, since  it  ceased  to  evolve  itself  from  the  leaves  in  dark- 
ness. Such  was  the  state  of  the  question  when  Sennebier  fully 
demonstrated  by  experiment,  that  the  oxygen  evolved  from  the 
leaves  was  the  result  of  their  decomposition  of  the  carbonic 
acid  which  was  contained  in  them. 

This  carbonic  acid  is  chiefly  abstracted  from  the  atmosphere 
by  means  of  the  stomata  or  pores  of  the  leaves.  Every  one 
must  be  aware  that  neither  plants  nor  animals  could  live  with- 
out air,  and  if  they  both  lived  on  the  same  air,  the  atmosphere 
would  soon  become  unfit  for  respiration.  But  the  air  taken 
into  the  lungs  of  animals  in  the  act  of  inspiration,  imparts  its 
oxygen  to  the  dark  venous  blood  in  the  lungs,  which  combining 
with, the  carbon  of  the  blood  forms  carbonic  acid ;  this  gas  is 
expelled  from  the  lungs  in  the  act  of  expiration  (ex,  out,  spiro9 
to  breathe.)  The  blood  thus  oxygenated  by  breathing,  loses 
its  dark  color  and  is  changed  into  that  bright  red  arterial 
stream  which  again  circulates  through  the  system  for  its  nutri- 
tion. Now  the  atmosphere  would  soon  be  thoroughly  poisoned 
by  animals,  but  for  the  purifying  influence  exerted  on  it  by 
the  vegetable  creation.  Carbonic  acid,  C02,  which  is  com- 
posed of  one  equivalent  of  carbon  and  two  equivalents  of 
oxygen,  is  taken  into  the  plant  through  the  pores  of  its  leaves, 
and  under  the  influence  of  solar  light  decomposed,  the  plant 
fixing  the  carbon,  which  when  thus  assimilated,  forms  the  chlo- 
rophyl  or  green  matter  in  them ;  whilst  the  oxygen  is  set  free 
and  escapes  into  the  atmosphere,  which  gas  is  the  food  of 
animals. 

This  inspiration  of  the  carbonic  acid  of  the  atmosphere, 


78  COMPOUND  ORGANS  OF  PLANTS. 

together  with  the  assimilation  of  the  carbon  and  the  expira- 
tion of  the  oxygen,  constitutes  what  may  be  truly  denominated 
vegetable  breathing  or  respiration. 

These  results  take  place  only  when  the  plant  is  exposed  to 
the  direct  rays  of  the  sun.  Recent  experiments  have  shown 
that  the  process  ceases  when  the  sun  is  behind  the  clouds,  and 
that  not  only  during  the  night,  but  even  under  the  influence 
of  diffused  daylight,  the  exhalation  of  oxygen  is  stopped. 

The  exhalation  of  oxygen  gas  from  the  leaves  of  plants  is 
the  only  provision  that  we  know  of  for  keeping  up  its  supply 
in  the  atmosphere.  The  prevailing  chemical  tendencies  are  to 
take  oxygen  from  the  air.  "Were  it  not  for  the  copious  sup- 
plies of  this  gas  poured  into  the  atmosphere  from  the  pores  of 
plants,  animal  life  could  not  exist.  Hence  the  perfect  adap- 
tation of  the  two  kingdoms  of  nature,  each  removing  from  the 
atmosphere  what  would  be  noxious  to  the  other,  each  yielding 
to  the  atmosphere  what  is  essential  to  the  life  of  the  other. 

To  show  that  plants  give  out  oxygen  in  sunshine.  Fill  a 
jar  with  water,  and  invert  it  in  a  vessel  containing  the  same 
fluid.  Introduce  beneath  the  jar  a  sprig  of  mint,  or  any  other 
living  plant.  After  a  while  bubbles  of  gas  will  collect  on  the 
leaves  and  ascend  to  the  top  of  the  jar,  displacing" the  water. 
If  the  air  thus  collected  be  tested,  it  will  be  found  to  be  pure 
oxygen  gas.  If  the  vessel  be  placed  in  the  shade,  the  bubbles 
of  gas  will  disappear  from  the  leaves. 

The  leaves  are  not  the  only  organs  of  vegetable  respiration. 
The  yojung  branches,  the  scales,  in  a  word,  all  the  herbaceous 
and  green  parts  of  plants  act  on  the  atmosphere  in  a  similar 
manner  to  the  leaves.  They  take  in  carbonic  acid  from  the 
atmosphere,  assimilate  the  carbon,  and  give  out  the  oxygen. 

Form  of  Leaves. — It  has  been  stated  that  the  leaf  of  a  plant 


THE   LEAVES.  79 

is  simply  an  expansion  of  the  wood  and  bark  of  its  stem,  the 
wood  issuing  from  the  side  of  the  shoot  whilst  in  its  green 
young  state  in  fibrous  bundles,  which  carry  with  them  at  the 
same  time  the  green  cellular  bark  of  the  shoot,  and  then  by 
their  expansion  spread  it  out  to  the  air  and  light  of  heaven. 
There  must,  therefore,  be  a  natural  adaptation  and  corres- 
pondence between  the  spread  of  the  woody  fibre  which 
constitutes  the  framework  of  the  leaf,  and  the  peculiarities 
of  its  form.  This  idea  was  first  suggested  by  Decandolle. 
According  to  him,  the  shape  of  leaves  depends  on  the  mode 
in  which  the  fibres  diverge  when  they  leave  the  side  of  the 
shoot,  and  upon  the  quantity  of  parenchyma  or  bark  which 
they  carry  with  them ;  and  by  him  this  arbitrary  nomenclature 
of  form  was  rendered  intelligible  and  reduced  to  something 
like  system  based  on  scientific  principles.  Decandolle  distin- 
guishes three  principal  modes  in  the  venation  of  leaves,  viz. : 
the  net-veined,  the  parallel-veined,  and  the  fork-veined. 

1.  The  reticulated  or  net-veined  leaves  are  characteristic  of 
exogens,  which  are  justly  regarded  as  the  most  highly  organ- 
ized plants  in  the  vegetable  world.     Two  modifications  of  net- 
veined  structure  have  been  observed,  the  feather-veined  and 
radiate-veined ;  the  leaves  of  the  chestnut  are  good  examples 
of  the  former,  and  those  of  the  garden  nasturtium  of  the  latter. 
The  margins  of  net-veined  or  exogenous  leaves  are  very  seldom 
entire,  but  most  frequently  notched  in  various  ways,  described 
in  books  as  dentate  or  toothed,  crenate  or  scolloped,  serrate  or 
having  teeth  like  a  saw,  of  which  last  we  have  a  good  example 
in  the  leaf  of  the  rose.     The  cause  of  these  incisions  has  not 
been  clearly  ascertained. 

2.  The  parallel-veined  leaves  are  the  distinguishing  feature 
of  endogens,  which  are  considered  humbler  in  their  organic 


80  COMPOUND   ORGANS   OF  PLANTS. 

structure.  That  nature  has  been  less  elaborate  in  their  forma- 
tion, will  be  evident  to  any  one  who  will  only  take  the  trouble 
to  compare  a  lily  leaf  with  that  of  a  rose.  If  held  up  to  the 
light,  the  intricate  and  highly  complex  ramifications  of  the 
fibrous  structure  of  the  exogenous  leaf  of  the  rose  will  be  seen 
in  striking  contrast  with  the  extreme  simplicity  of  the  endo- 
genous leaf  of  the  lily.  Two  different  modes  of  venation  have 
also  been  noted  in  endogens,  the  curve-veined  and  the  straight- 
veined.  In  the  first  instance,  the  veins  run  in  parallel  curves 
from  the  base  to  the  apex  of  the  leaf,  and  in  the  other  case 
proceed  in  right  lines.  The  plantain  and  Hemerocallis  or 
day-lily,  are  good  examples  of  the  first,  and  grasses  of  the  last 
method  of  venation.  The  margin  of  endogenous  leaves  is  inva- 
riably entire,  and  never  marked  with  indentations  of  any  kind. 
3.  The  fork-veined  leaves,  which  are  peculiar  to  ferns,  plants 
still  lower  in  the  scale  of  organization.  It  may  be  proper  to 
qualify  these  divisions  and  sub-divisions,  by  remarking  that 
they  are  not  intended  accurately  to  define  the  boundaries 
between  the  different  modes  of  venation.  There  is  an 
approach  to  the  forked  method  of  venation  in  some  exo- 
genous plants,  as  in  clover,  and  doubtless  there  are  many 
intermediate  forms.  All  classification  is  but  an  approxima- 
tion to  that  order  which  obtains  in  nature.  All  that  Decan- 
dolle  intended,  was  to  point  out  some  of  the  principal  modes 
in  which  the  woody  matter  of  leaves  was  distributed  through 
their  parenchyma,  and  to  call  attention  to  the  fact  that  the 
variety  of  their  form  is  the  result  of  one  or  the  other  of 
these  modes  of  distribution.  The  student  will  now  under- 
stand that  leaves  assume  the  linear,  lanceolate,  ovate  or 
orbicular  form,  according  to  the  greater  or  less  degree  of 
divergence  of  the  woody  fibre  constituting  their  framework. 


THE   LEAVES.  81 

Simplicity  in  causes  and  variety  in  effects  mark  all  the  opera- 
tions of  nature  ! 

The  distribution  of  leaves  about  the  stem. — All  who  notice 
plants  much  have  frequently  observed  the  regularity  and  sym- 
metry with  which  leaves  are  arranged  around  the  stem.  Some- 
times they  spring  from  its  sides  in  pairs,  crossing  each  other  at 
right  angles,  as  in  the  mint  family,  or  in  beautiful  whorls,  as 
in  the  Galium  or  bedstraw  tribe ;  and  again,  they  are  scattered 
along  the  stem  on  either  side,  but  still  with  an  apparent  regu- 
larity, and  certainly  not  at  random.  These  peculiarities  of 
their  distribution  are  produced  by  a  combination  of  the  two 
following  causes. 

1.  The  manner  in  which  the  stem  grows.  If  the  elongation 
of  the  stem  and  the  growth  of  the  leaves  be  simultaneous,  the 
leaves  will  be  scattered  on  all  sides  of  the  stem,  and  will  be 
few  or  numerous,  according  to  the  greater  or  less  degree  of 
rapidity  with  which  they  are  developed ;  but  if  the  elongation 
of  the  stem  is  periodically  checked,  and  the  growth  of  the 
leaves  at  the  same  time  continues,  they  will  necessarily  start 
out  from  the  same  point  of  the  stem  in  pairs  or  in  whorls, 
according  to  the  length  of  time  taken  up  before  the  stem 
again  elongates.  This  is  well  seen  in  Lysimachia  quadrifolia, 
which  in  ordinary  circumstances  bears  whorls  of  four  and  six 
leaves ;  these,  when  the  growth  of  the  stem  is  rapid,  become 
alternate.  We  have  also  instances  of  the  operation  of  this  law 
in  the  Coniferae,  or  pine '  family.  The  Larch  has  leaves  de- 
veloped in  fascicles  or  bundles.  These  leaves  are  without  any 
lamina  or  blade,  rigid  and  needle-shaped  or  linear.  They  are 
brought  together  in  consequence  of  their  rapid  development 
and  the  non-elongation  of  their  axis  of  growth.  That  this  is 
really  the  cause  of  their  fascicled  character,  is  evident  on  close 

8 


82  COMPOUND  ORGANS  OF  PLANTS. 

inspection  of  the  young  shoots  of  the  larch,  which  by  their 
rapid  growth  do  not  admit  of  any  fascicular  development  of 
their  leaves.  On  these  shoots  the  young  leaves  of  the  larch 
will  be  found  to  be  scattered,  not  fascicled,  clearly  showing 
their  natural  arrangement,  and  proving  that  the  fascicles  are 
the  result  of  the  development  of  the  leaves  and  the  non-de- 
velopment of  the  nodes  of  the  stem. 

The  spired  growth  of  the  leaves.  This  is  most  readily 
perceived  in  such  plants  as  have  their  leaves  distributed 
alternately  on  either  side  of  the  stem.  If  a  thread  be  wound 
about  the  stem  so  as  to  touch  the  basis  of  the  first,  second, 
third,  fourth,  and  succeeding  leaves,  it  will  be  found  to  de- 
scribe an  ascending  spiral  around  the  stem,  and  with  such 
accuracy  that  the  law  may  be  expressed  numerically.  The 
observations  of  Dr.  Gray,  on  leaf  arrangement,  are  too 
interesting  to  be  omitted  in  this  place.  "  If  we  write  down 
in  order  the  series  of  fractions  which  represent  the  simpler 
forms  of  leaf  arrangement,  as  determined  by  observation,  viz. : 

3>  i>  §»  l>  T5s>  28T>  il>  we  can  nardly  fail  to  perceive  the 
relation  that  they  bear  to  each  other.  For  the  numerator  of 
each  is  composed  of  the  sum  of  the  numerators  of  the  two 
preceding  fractions,  and  the  denominator  of  the  sum  of  the  two 
preceding  denominators.  Also,  the  numerator  of  each  fraction 
is  the  denominator  of  the  next  but  one  preceding.  We  may 
carry  out  the  series  by  applying  this  simple  law,  when  we 
obtain  the  farther  terms,  T53,  a^vj},  fj,  &c.  Now  these 
numbers  are  those  which  are  actually  verified  from  observation, 
and,  with  some  abnormal  exceptions,  this  series  comprises  all 
the  cases  that  occur/' 

That  the  interest  which  attaches  to  the  above  extract  may 
be  fully  appreciated,  I  remark,  that  the  fractions  severally 


THE   LEAVES.  83 

represent  different  kinds  of  spirals,  the  numerator  denoting  the 
number  of  times  that  the  thread  winds  round  the  stem  before 
it  touches  the  base  of  a  leaf  directly  over  the  one  it  began 
with,  whilst  the  denominator  expresses  the  number  of  leaves 
it  touches  in  its  course,  before  it  arrives  at  that  leaf  thus 
situated.  Thus  the  fraction  £  denotes  that  the  thread  winds 

5 

twice  round  the  stem,  covering  the  bases  of  five  leaves  in  its 
course ;  consequently,  the  sixth  leaf  stands  directly  over  the 
first. . 

But  the  most  curious  and  wonderful  thing  is  that  the  higher 
fractions  _^,  28T,  &c.,  as  developed  by  the  application  of  this 
numerical  law,  are  positively  realized  in  nature.  For  the 
same  principle  of  arrangement  -  extends  to  all  those  parts  of 
plants  which  are  modifications  of  leaves,  and  these  numbers 
are  actually  verified  when  we  come  to  examine  the  rosettes  of 
the  houseleek,  and  the  scales  of  pine  cones.  It  is  the  com- 
bination of  both  these  causes,  the  tendency  to  spiral  develop- 
ment, combined  with  the  peculiarities  of  stem  growth,  which 
disposes  the  leaves  of  plants  with  so  much  regularity  and 
symmetrical  beauty  around  their  stems. 

Leaves  sometimes  assume  very  curious  forms. — Sometimes 
the  lamina  or  thin  expanded  portion  of  the  leaf  become.s  nearly 
or  altogether  abortive,  and  the  petiole  itself  assumes  a  leaf-like 
appearance.  This  modification  of  structure  is  termed  a  phyllo- 
dium.  The  leaves  of  the  New  Holland  acacias  are  all  more 
or  less  formed  into  phyllodia.  These  plants  have  compound 
pinnate  leaves,  and  just  in  proportion  as  the  pinnae  of  the  limb 
are  suppressed,  is  their  petiole  expanded  and  leaf-like.  In 
young  acacias,  and  occasionally  in  old  ones  which  have  been 
freely  pruned,  all  the  intermediate  states  between  a  compound 
pinnate  leaf  and  a  simply  expanded  petiole  may  be  observed. 


84  COMPOUND  ORGANS  OF  PLANTS. 

Decandolle  considers  that  the  sheathing  leaves  of  endogenous 
plants  which  are  not  furnished  with  a  distinct  limb,  are  only 
expanded  petioles.  The  leaves  of  the  Hyacinth,  and  Iris 
versicolor,  or  common  blue  flag  of  the  pools,  are  of  this  nature. 
Such  leaves  are  sometimes  met  with  even  in  the  higher  order 
of  exogenous  plants,  as,  for  instance,  Ranunculus  flammula  or 
Spearwort  crowfoot,  a  common  aquatic  plant. 

Sometimes  the  edges  of  the  lamina  or  blade  cohere  together, 
producing  still  stranger  modifications  of  leaf  structure.  It  is 
well  known  that  the  parts  of  plants  which  grow  closely 
together,  are  apt  to  become  coherent.  Accidental  unions  of 
this  kind  amongst  the  leaves  of  plants  are  of  common  occur- 
rence. In  some  species  these  unions  occur  after  the  plant  is 
considerably  grown,  as  in  the  garden  honeysuckle,  the  upper 
leaves  of  which  usually  cohere  together  by  their  bases,  owing 
to  their  sessile  character,  and  form  what  botanists  call  a  connate 
leaf.  So  also,  the  numerously  crowded  and  closely  compact 
leaves,  constituting  the  calyx  or  cup  of  the  Marygold  or  Holly- 
hock flowers,  will  be  found  to  be  more  or  less  united  with  each 
other.  In  other  instances,  where  the  cohesion  of  the  leaves 
with  each  other  or  with  the  stem  is  of  constant  occurrence  in 
every  stage  of  vegetable  development,  this  union  appears  to 
take  place  at  a  much  earlier  period.  In  this  case,  whilst  the 
plant  lies  folded  up  within  the  seed  and  its  texture  is  yet  deli- 
cate, the  numerous  vessels  of  its  organs  which  are  thus  brought 
into  close  contact  anastomose ;  that  is,  run  together  or  unite  with 
one  another  by  means  of  the  elaborated  juices  which  nourish 
them,  thus  producing  those  cohesions  of  the  parts  of  plants  which 
are  visible  in  their  after  developments. 

If  these  views  be  correct,  they  will  serve  to  explain  the 
nature  of  the  hollow  leaves  of  Sarracenia  purpurea,  or  the  side 


THE    LEAVES.  85 

saddle  flower,  with  its  leafy  cups  half  filled  with  water  and 
dead  insects,  which  abounds  in  the  bogs  of  the  Northern  and 
Middle  States.  This  pitcher  may  be  conceived  to  be  formed 
by  the  cohesion  of  the  edges  of  a  partly  formed  phyllodium. 
If  we  imagine  a  dilated  petiole  with  its  partially  formed  lamina 
to  curve  over  and  unite  at  its  edges,  a  leaf  like  that  of  the 
Sarracenia  will  evidently  be  formed,  in  which  the  pitcher  will 
be  simply  a  hollow  petiole,  whilst  the  hood  at  its  summit  is 
produced  by  its  abortive  lamina  or  blade. 

In  Utricularia,  or  bladderwort,  the  leaves  form  sacs  called 
ampulla,  which  are  filled  with  air,  and  float  the  plant  in  the 
water  at  the  time  of  flowering. 

But  the  most  remarkable  case  of  leaf  cohesion,  is  seen  in  the 
Nepenthes  distillatoria,  or  pitcher  plant  of  the  East  Indies.  In 
this  instance,  the  petiole  when  it  first  leaves  the  side  of  the  stem, 
is  round,  or  of  its  usual  shape,  then  it  expands  into  a  leaf-like  organ 
or  phyllodium,  and  next  it  is  contracted  into  a  tendril,  finally  it 
forms  into  a  phyllodium,  the  sides  of  which  cohere  together  so  as  to 
form  a  pitcher,  which  is  surmounted  at  the  summit  by  the 
abortive  lamina  or  blade,  in  the  shape  of  a  lid.  This  pitcher 
is  constantly  filled  with  about  half  a  pint  of  pure  water,  which 
is  not  collected  from  without,  as  in  the  Sarracenia,  but  is 
secreted  by  the  plant :  for  the  lid  surmounting  their  summit 
constantly  and  accurately  closes  the  orifice  of  these  pitchers, 
and  their  internal  surface  is  of  a  glandular  structure.  In 
Ceylon,  where  this  plant  is  common,  it  is  called  by  the  natives 
by  a  word  the  signification  of  which  is  monkey  cup ;  because 
these  cunning  animals  when  thirsty,  and  there  is  no  stream  at 
hand,  open  the  lid  and  drink  the  contents.  Men  also  travelling 
or  hunting  in  the  woods,  often  find  the  water  contained  in  these 
vegetable  pitchers  a  means  of  assuaging  their  thirst, 
x  8* 


86  COMPOUND  ORGANS  OF  PLANTS. 

The  fall  of  the  leaf. — There  is  no  subject  on  which  botanists 
have  entertained  a  greater  variety  of  opinion  than  on  the  fall 
of  leaves.  The  causes  which  produce  their  excision  from  the 
stems  and  branches  of  plants  are  so  exceedingly  complicated, 
that  a  much  more  advanced  condition  of  botanical  science  seems 
to  be  necessary  before  they  will  be  clearly  and  accurately 
understood.  It  is  obvious  that  leaves  are  thrown  off  by  plants 
because  they  are  no  longer  of  any  service  to  them,  and  the 
means  by  which  nature  effects  their  separation  are  truly  won- 
derful, and  at  the  same  time  instructive. 

The  causes  which  produce  the  decay  and  fall  of  leaves  are 
partly  chemical  and  mechanical.  The  water  which  enters  the 
roots  of  plants  as  it  percolates  the  soil,  dissolves  a  small  portion 
of  earthy  matter.  This  is  partly  deposited  in  the  woody  and 
fibrous  tissues  of  the  stem,  but  principally  in  the  cellular  tissue 
of  the  leaves,  by  the  evaporation  which  is  continually  taking 
place  at  their  surface.  In  this  manner  the  interior  walls  of  the 
leaf  cells  become  encrusted  or  thickened  by  deposits  of  mineral 
matter,  just  as  earthy  matter  accumulates  at  the  bottom  of  a 
pot  used  for  culinary  purposes,  and  the  leaf  is  thus  rendered 
finally  unfit  for  the  performance  of  its  functions.  The  mineral 
matter  deposited  in  the  cells  is  sometimes  beautifully  crystal- 
lized, the  earths  or  bases  taken  up  by  the  roots  uniting  with  the 
acids  formed  in  the  vegetable  organs.  The  most  common  kinds 
of  crystals  are  those  of  the  carbonate  and  oxalate  of  lime  which 
are  of  different  sizes  and  forms,  rhomboidal,  cubical  and  pris- 
matic ;  but  the  most  prevalent  form  is  the  acicular  or  needle- 
shaped.  It  is  to  this  form  that  the  term  raphides  (raphis  a 
needle)  was  originally  applied  by  Decandolle,  although  it  is 
now  used  indiscriminately  in  reference  to  all  cellular  crystals. 

In  the  autumnal  months,  the  light  becomes  less  powerful,  the 


THE   LEAVES.  87 

leaves  lose  their  green  color,  and  their  cells  becoming  gradually 
and  entirely  choked  up  with  mineral  matter,  the  sap  no  longer 
circulates  through  them.  They  absorb  oxygen  from  the  air, 
and  the  result  of  their  different  degrees  of  oxidation  is  seen  in 
all  that  variety  of  autumnal  tint,  which  casts  such  a  charm  over 
the  dying  landscape. 

Whilst  these  chemical  changes  are  taking  place,  nature  is  at 
the  same  time  preparing  to  effect  the  mechanical  excision  of  the 
leaf  from  the  plant. 

Now,  at  first,  all  leaves  are  contiguous  with  the  stem.  As 
they  grow,  an  interruption  of  their  tissue  takes  place  at  the 
base  of  their  footstalk,  by  means  of  which  a  more  or  less  com- 
plete articulation  is  formed.  This  articulation  is  produced  by 
the  continuation  of  the  growth  of  the  stem  after  the  leaf  has 
attained  its  full  growth,  which  it  generally  does  in  a  few  weeks. 
The  growth  of  the  leaf  being  completed,  all  its  functions  lan- 
guish in  consequence  of  the  increased  deposition  of  mineral 
matter  within  its  cells,  and  the  base  of  the  petiole  or  footstalk 
being  no  longer  able  to  adapt  itself  to  the  increasing  diameter 
of  the  stem,  a  fracture  between  the  base  and  stem  necessarily 
ensues ;  the  excision  advances  from  without  inwards,  until  it 
finally  reaches  the  bundles  of  woody  fibre,  which  are  the  main 
support  of  the  leaf. 

Whilst,  however,  nature  is  forming  a  wound,  she  is  at  the 
same  time  making  provision  to  heal  -the  same  j  for  the  cuticle 
or  epidermis  of  the  stem  is  seen  to  grow  over  the  surface  of 
the  scar,  so  that  when  the  leaf  is  detached  the  tree  does  not 
suffer  from  the  effects  of  an  open  wound.  The  provision  for 
separation  being  thus  completed  the  leaf  is  detached  by  the 
growth  of  the  bud  at  its  base,  by  the  force  of  the  winds,  or 
even  by  its  own  weight.  Such  is  the  philosophy  of  the  fall  of 


88  COMPOUND  ORGANS  OF  PLANTS. 

leaves,  and  we  cannot  help  admiring  the  interesting  and  won- 
derful provision  by  which  nature  heals  the  wounds  even  before 
they  are  absolutely  made,  and  affords  a  safe  covering  from 
atmospheric  changes  before  the  parts  can  be  subject  to  them. 

The  decay  and  fall  of  leaves  is,  therefore,  not  the  result  of 
frost,  as  is  commonly  supposed,  for  leaves  begin  to  languish 
and  change  color  (as  happens  with  the  red  maple,  especially,) 
and  even  fall,  often  before  the  autumnal  frosts  make  their 
appearance,  and  when  vegetation  is  destroyed  by  frost  the 
leaves  blacken  and  wither  but  remain  attached  to  the  stem ;; 
but  the  death  and  fall  of  the  leaf  is  produced  by  a  regular 
vital  process,  which  commences  with  the  first  formation  of  this 
organ,  and  is  completed  only  when  it  is  no  longer  useful. 
There  is  no  denying,  however,  that  the  frosts  of  autumn,  by 
suddenly  contracting  the  tissues  at  the  base  of  the  petiole, 
accelerate  the  fall  of  leaves.  All  must  have  noticed,  on  a 
frosty  morning  in  autumn,  that  the  slightest  breath  of  air 
moving  amongst  the  decayed  and  dying  leaves,  will  bring  them 
in  complete  showers  from  the  trees  to  the  ground. 

In  general,  we  may  say,  that  the  duration  of  life  in  leaves  is 
inversely  as  the  force  of  the  evaporation  which  takes  place 
from  their  surface.  For  we  find  that  the  leaves  of  herbaceous 
plants,  or  of  trees  which  evaporate  a  great  deal,  fall  before  the 
end  of  the  year,  whilst  the  leaves  of  succulent  plants,  or  of 
evergreens,  which  latter  are  of  a  hard  and  leathery  texture,  and 
evaporate  but  little,  often  last  for  several  years.  In  pines, 
firs,  and  evergreen  trees  and  shrubs,  there  is  an  annual  fall  of 
leaves  in  the  spring  of  the  year  whilst  the  growth  of  the 
season  is  taking  place ;  but  as  this  leaf-fall  is  only  partial,  con- 
sisting of  one-half  or  one-third  at  a  time,  there  is  always  a 
sufficient  number  left  on  such  trees  to  keep  them  clothed  with 


NATURE   AND    SOURCES   OF  FOOD.  89 

perpetual  verdure.  Hence  it  is,  that  the  entire  foliage  of  such 
trees  consists  of  leaves  which  have  been  attached  to  the  stem 
from  one  to  three  or  five  successive  years. 

In  the  beech  and  hornbeam,  the  leaves  wither  in  autumn, 
and  hang  on  the  branches  in  a  dead  state  through  the  winter. 
Such  leaves,  when  examined,  will  be  found  to  be  contiguous 
with  the  stem  at  the  base  of  their  petiole,  and  therefore  with- 
out that  articulation  or  joint  which  so  materially  aids  in  the 
disruption  of  the  leaf  from  the  stem.  These  dead  leaves  fall 
off  when  the  new  leaves  expand  in  the  spring. 

Most  of  the  trees  of  this  country  have  deciduous  leaves,  and 
in  winter  our  woods  are  bare  and  no  longer  cast  their  shadows 
on  the  earth  ;  but  the  forests  of  tropical  climates  are  evergreen, 
and  usually  retain  the  same  appearance  throughout  the  year. 
A  perpetual  shade  is  thus  afforded  by  nature,  which  in  some 
measure  gives  relief  against  the  continuous  heat  of  these 
regions. 


CHAPTER    VII. 

ON  THE  NATURE  AND  SOURCES  OF  THE  FOOD  ASSIMILATED 
BY  PLANTS. 

THE  investigation  of  the  nature  and  sources  of  those  sub- 
stances assimilated  by  the  nutritive  organs  of  plants,  is  neces- 
sary to  a  clear  understanding  of  their  physiological  action,  and 
will  very  properly  close  this  part  of  the  subject. 

These  substances  can  only  be  determined  by  chemical  analy- 
sis. Plants  have  been  examined  chemically  by  Liebig,  Mulder, 


90  COMPOUND   ORGANS   OF   PLANTS. 

and  Johnson,  and  we  are  about  to  lay  before  the  student  the 
results  of  the  labors  of  these  philosophers. 

The  solid  part  of  plants,  chemically  considered,  is  found  to 
consist  of  organic  and  inorganic  matter;  the  first  may  be  burnt 
away  and  is  derived  from  the  atmosphere,  the  second  is  incom- 
bustible and  is  derived  from  the  soil  in  which  the  plant  grows. 

To  show  the  organic  and  inorganic  matter  in  plants.  Burn 
a  piece  of  wood  or  straw  in  the  flame  of  a  lamp.  The  part 
which  burns  is  organic  matter  and  passes  again  into  the  atmo- 
sphere from  whence  it  was  taken;  the  incombustible  ash  that 
remains  is  the  inorganic  matter  in  the  plant,  which  was  derived 
from  the  soil. 

The  organic  part  of  plants  is  composed  of  four  substances, 
carbon,  or  charcoal,  more  than  one-half,  oxygen  one-third, 
hydrogen  one-twentieth,  and  nitrogen  one-fiftieth. 

The  inorganic  part  of  plants,  or  the  ash  remaining  after  the 
combustion  of  the  organic  matter  in  them,  consists  of  no  less 
than  eleven  different  substances,  viz  :  potash,  soda,  lime,  mag- 
nesia, silica,  oxide  of  iron,  oxide  of  manganese,  sulphur,  sul- 
phuric acid,  phosphoric  acid,  and  chlorine. 

The  carbon  or  charcoal  in  plants  composes  more  than  one- 
half  of  their  entire  bulk.  If  a  green  leaf  or  a  piece  of  wood 
be  charred  (which  may  be  done  by  heating  it  in  a  close  vessel 
out  of  contact  with  the  air,)  all  the  hydrogen  and  oxygen  in 
the  plant  will  be  driven  off,  and  what  remains  will  be  the 
amount  of  carbon  in  the  plant,  together  with  a  small  per- 
centage of  inorganic  matter.  The  leaf  or  specimen  of  wood 
which  has  been  thus  carbonized  will  be  found  to  preserve  its 
form  and  bulk  uninjured,  even  to  that  of  the  most  delicate 
cells  and  vessels,  but  will  be  considerably  lighter.  A  piece  01 
common  stove  charcoal  is  a  beautiful  instance  of  wood  which 


NATURE   AND   SOURCES   OP   FOOD.  91 

has  been  thus  treated,  and  evinces  that  charcoal  is  the  principal 
constituent  in  the  material  out  of  which  a  plant  is  constructed. 

The  carbon  found  in  plants  is  derived  from  the  atmosphere 
and  from  the  decomposing  vegetable  matter  in  the  soil.  It  has 
been  shown  how  plants  take  in  carbon  from  the  atmosphere, 
through  the  pores  of  their  leaves,  in  the  form  of  carbonic  acid 
gas.  But  the  atmosphere  is  not  the  only  source  ;  the  soil  also 
contains  an  immense  quantity,  for  carbonic  acid  is  given  off  not 
only  by  the  lungs  of  animals,  but  by  burning  bodies,  and  by 
the  decaying  animal  and  vegetable  matter  in  the  soil. 

When  we  burn  a  plant  and  thus  effect  a  separation  between 
its  organic  and  inorganic  constituents,  restoring  the  former 
to  the  atmosphere  and  isolating  the  latter  under  the  form  of 
ashes,  the  process  of  combustion  is  only  the  result  of  the  rapid 
union  of  the  oxygen  of  the  air  with  the  carbon  in  the  leaf,  and 
the  consequent  formation  of  carbonic  acid  gas.  Now  precisely 
the  same  process  occurs  in  nature  when  plants  decay  and  disap- 
pear from  the  earth's  surface. 

The  decay  of  vegetable  bodies  in  the  soil,  as  Liebig  has 
shown,  is  only  a  slower  process  of  combustion,  being  produced 
by  precisely  the  same  cause,  viz.,  the  union  of  the  oxygen  of 
the  air  with  the  carbon  in  the  plant,  with  the  consequent  pro- 
duction of  carbonic  acid  gas. 

Hence  we  see  the  reason  why  wood  when  it  gradually 
decays  becomes  brown  and  ultimately  black,  presenting  pre- 
cisely the  same  appearance  as  if  it  had  been  burnt  with  fire. 

In  the  process  of  decay,  or  as  it  is  termed  chemically, 
eremacausiSj  that  is  slow  burning,  the  oxidation,  of  the  vege- 
table is  so  slow  that  neither  heat  nor  light  is  evolved ;  hence 
the  products  of  the  vegetable  decomposition  are  aqueous  as 
well  as  gaseous,  or  the  body,  popularly  speaking,  putrefies. 


92  COMPOUND  ORGANS  OF  PLANTS. 

To  this  decaying  animal  and  vegetable  matter  the  term  humus 
is  applied.  It  constitutes  the  brown  or  black  portion  of  every 
soil.  Wherever  it  exists,  there  plants  spring  up  the  most  readily, 
whilst  in  places  devoid  of  it,  they  are  stunted  and  dwarfed  in 
their  growth  and  decidedly  inferior  both  in  organization  and 
beauty.  Thus,  though  carbonic  acid  is  principally  absorbed 
from  the  air  by  the  leaves,  the  roots  of  plants  also  find  it  in 
every  soil  which  contains  humus;  for  humus  consists  in 
decaying  organic  matter,  that  is,  organic  matter  resolving  itself 
by  a  sort  of  slow  combustion  into  carbonic  acid  and  water. 

Carbonic  acid  makes  up,  on  the  average,  only  one  two- 
thousandth  part  of  the  bulk  of  the  atmosphere.  It  is, 
however,  very  soluble  in  water,  and  its  accumulation  in  the 
air  like  that  of  ammonia  is  mainly  prevented  by  the  rains 
which  greedily  absorb  and  wash  it  down  to  the  'earth,  from 
whence  it  is  imbibed  by  the  root.  In  this  manner  carbonic 
acid  enters  the  system  of  the  plant  by  the  roots  as  well  as  by 
the  leaves. 

Hydrogen  and  the  greater  part  of  the  oxygen  enter  the 
plant  by  the  roots  in  the  form  of  water  (ELO),  which  consists 
of  these  two  gases  in  chemical  union.  These  two  gases  indis- 
solubly  bound  together  in  the  form  of  water,  which  circulating 
through  nature  on  entering  the  system  of  plants,  is  neverthe- 
less readily  decomposed  by  the  powers  of  vitality. 

Nitrogen  enters  by  the  roots  chiefly  in  the  form  of  nitric 
acid  and  ammonia.  The  former  is  produced  during  the  passage 
of  electricity  through  the  air ;  the  latter  is  copiously  evolved 
from  compost  heaps  and  from  decaying  vegetable  and  animal 
matter. 

To  test  the  presence  of  ammonia  in  the  compost  heap.  Dip 
a  glass  tube  in  hydrochloric  acid  (spirit  of  salts)  and  hold  it 


NATURE   AND    SOURCES   OP   FOOD.  93 

over  the  heap.  If  ammonia  be  present,  copious  white  fumes 
will  be  perceived,  which  result  from  the  chemical  union  of  the 
hydrochloric  acid  gas  with  the  ammoniacal  gas,  and  the  forma- 
tion of  a  salt,  the  hydrochlorate  of  ammonia,  or  the  sal 
ammoniac  of  the  stores.  Wf*q  Cc 

Although  ammonia  is  constantly  rising  in  vast  quantities 
into  the  atmosphere  from  decaying  animal  and  vegetable 
matter,  it  is  nevertheless  easily  soluble  in  water,  and  is  there- 
fore prevented  from  accumulating  there  by  the  aqueous  vapor 
of  the  atmosphere,  which,  when  it  is  precipitated  thence  in  the 
form  of  rain,  conveys  the  ammonia  in  solution  to  the  roots  of 
plants.  That  this  is  the  fact  is  evident  because  ammonia  can 
be  detected  in  rain  water  and  in  the  sap  of  plants,  and  also 
because  all  manures  such  as  guano,  which  contain  a  great 
amount  of  ammonia,  are  found  to  be  fertilizing  to  soils. 

The  combustible  or  organic  part  of  the  plant  forms  by  far 
the  greater  part  of  its  structure.  This  is  evident  from  the 
small  amount  of  ash  or  inorganic  matter  left  after  its  incinera- 
tion. It  follows  that  plants  derive  the  materials  of  their 
growth  mainly  from  the  atmosphere. 

That  certain  plants  derive  the  greater  part  of  their  food 
from  the  atmosphere,  affords  an  explanation  of  the  process  by 
which  nature  changes  the  barren  rock  into  the  fertile  soil. 
The  first  plants  which  clothe  the  surface  of  the  newly  formed 
coral  reef,  or  of  our  common  rocks,  are  lichens  and  mosses ; 
plants  which  derive  the  greater  part,  if  not  the  whole  of  their 
nutriment,  entirely  from  the  atmosphere.  Now  plants  can  only 
grow  in  proportion  to  the  quantities  of  food  afforded  them. 
Lichens  and  mosses  are  plants  of  very  humble  growth  and 
exceedingly  simple  structure,  consisting  of,  comparatively 
speaking,  only  a  few  cells.  Successive  generations  of  these 

9 


94  COMPOUND  ORGANS  OF  PLANTS. 

atmospheric  cryptogamia  flourish  and  die,  forming  a  humus 
for  the  growth  of  grasses,  ferns,  and  more  highly  organized 
plants ;  until  at  length  there  is  formed  on  the  surface  of  that 
once  barren  rock  a  sufficiency  of  humus  for  the  nutrition  of  all 
the  varieties  of  vegetable  organisation  found  in  the  fertile 
meadow,  the  tangled  thicket,  and  the  widely  extended  forest. 
Finally  man  comes  to  take  possession  of  the  new  domain 
which  nature  has  thus  been  carefully  preparing  for  him,  and 
life  reaches  its  highest  stage  of  development. 

The  inorganic  matter  constituting  the  ash  which  remains  after 
the  combustion  of  the  plant,  is  wholly  absorbed  from  the  soil, 
and  enters  the  plant  in  a  state  of  solution  by  the  pores  of  the 
roots.  Some  persons  have  supposed  that  these  mineral  matters 
were  produced  by  the  plants  themselves,  and  not  derived  from 
without.  It  is  true  that  the  earths,  such  as  silica  or  sand, 
alumina  or  clay,  are  insoluble  by  themselves  in  water,  and  that 
the  subdivision  of  the  matter  of  which  they  are  composed  must 
be  carried  to  an  almost  infinite  degree  of  minuteness,  before 
they  can  pass  into  the  system  of  the  plant  through  the  minute 
pores  of  the  roots;  but  all  the  earths  are  soluble  with  the 
alkalies,  such  as  potash,  which  enters  largely  into  the  composi- 
tion of  all  rocks,  and  as  these  earths  are  furnished  to  the  soil 
by  the  slow  decomposition  or  disintegration  of  rocks,  there  can 
be  no  doubt  that  the  water,  as  it  percolates  the  soil  impreg- 
nated with  potash  and  carbonic  acid,  effects  their  solution  to 
such  an  extent  that  they  pass  unimpeded  into  the  system  of 
the  plant  along  with  the  water  which  is  imbibed  by  the  root. 

Each  species  of  plant,  according  to  its  peculiar  constitution, 
retains  a  greater  or  less  amount  of  one  or  more  of  these  earthy 
ingredients.  Thus,  nearly  all  plants  retain  a  quantity  of 
potash ;  wheat,  a  certain  amount  of  silex ;  some  aquatic  plants 


NATURE   AND    SOURCES   OF   FOOD.  95 

accumulate  iron  so  that  on  decaying  they  leave  a  sediment  of 
iron  rust  in  the  water ;  chlorine  is  found  in  all  marine  plants ; 
phosphorus  in  the  onion ;  and  sulphur  in  mustard  seed,  in 
celery,  and  in  ginger.  The  immense  quantities  of  water  vari- 
ously impregnated  with  these  foreign  bodies,  which  pass 
through  a  plant,  being  condensed  by  evaporation  in  the  leaves, 
is  sufficient  to  account  for  their  presence,  in  appreciable 
quantities  in  the  plant,  however  minute  may  be  their  propor- 
tion in  the  water  which  the  roots  imbibe.  Hence  it  is  found 
that  plants  will  not  grow  in  distilled  water,  or  water  freed  from 
all  foreign  ingredients ;  and  also  that  the  water  exhaled  by 
plants  is  so  pure  that  not  a  trace  of  foreign  matter  is  discover- 
able in  it ;  the  stomata  or  pores  of  the  leaves  are  in  fact  the 
most  perfect  stills  in  the  great  laboratory  of  nature.  About 
two-thirds  of  the  fluid  taken  up  by  the  spongioles  of  the  roots, 
is  evaporated  from  the  leaves  of  plants  in  the  form  of  water, 
and  consequently  about  one-third  remains  in  the  plant  in  a 
highly  concentrated  state,  and  contains  the  carbonic  acid  and 
other  earthy  ingredients  which  happen  to  be  dissolved  in  the 
fluid  when  first  presented  to  the  roots.  - 

Although  the  ash  or  inorganic  matter  in  plants  constitutes  a 
very  small  proportion  of  their  substance,  yet  its  importance  is 
not  on  this  account  to  be  underrated.  The  small  per  centage 
of  inorganic  matter  contained  in  them  appears  to  be  absolutely 
necessary  to  their  healthy  growth.  It  is  for  this  reason  that 
the  soil  exercises  such  a  marked  influence  on  the  distribution 
of  species.  It  is  impossible  to  examine  the  plants  which  spring 
up  spontaneously  in  any  district,  without  arriving  at  the  conclu- 
sion that  they  are  influenced  in  the  development  of  the  pecu- 
liarities of  their  organization,  by  certain  inorganic  matters  which 
abound  in  the  soils  in  which  they  grow.  The  barren  mountain 


96  COMPOUND  ORGANS  OF  PLANTS. 

and  the  fertile  valley,  the  sandy  soil  and  the  marshy  swamp, 
the  margin  of  rivers  and  shores  of  the  ocean,  have  all  their 
peculiar  species  of  plants.  The  chemical  composition  of  the 
ash  of  a  plant  being  known,  scientific  conclusions  can  be  drawn 
as  to  the  soil  most  suitable  for  its  growth. 

A.  good  soil  must  contain  all  the  substances  found  in  the  ash 
of  the  plant.  This  is  a  matter  of  great  importance  to  the  agri- 
culturist. If  we  give  abundant  and  vigorous  food  to  an  animal 
it  becomes  strong  and  fat ;  if  its  food  be  small  in  quantity  and 
poor  in  quality,  it  becomes  poor  and  lean.  Just  the  same 
happens  to  a  plant.  Plants  will  grow  vigorously  and  fruit 
plentifully  when  there  is  an  abundance  of  that  kind  of  food  in 
the  soil  which  is  the  most  suited  to  their  organization;  and 
their  growth  will  be  checked  and  their  fruit  injured  by  any 
deficiency  in  their  proper  food.  Nature  is  a  wise  and  perfect 
cultivator.  Some  plants  are  found  in  a  moist  soil,  others  in  a 
dry  one.  Some  seek  the  cool  shade,  others  the  warm  sunshine ; 
some  are  natives  of  lofty  and  barren  mountains,  others  of  lowly 
and  fertile  valleys ;  some  fixed  to  rocks  delight  in  the  noisy 
waves  of  the  sea;  others  attached  to  stones  in  brooks  and 
rivers  grow  beautifully  in  their  quiet  waters.  All  plants,  how- 
ever, are  placed  by  nature  in  soils  which  are  chemically  and 
physically  adapted  to  promote  their  growth,  so  that  they  may 
answer  her  grand  and  secret  purposes  in  the  development  of 
their  organization. 

The  motion  of  the  sap  in  plants. — The  function  of  nutrition, 
which  -in  the  higher  animals  comprises  a  variety  of  distinct 
processes,  is  reduced  in  plants  to  the  utmost  degree  of  simpli- 
city. When  water  charged  with  nutritive  substances  from  the 
soil  enters  the  cellular  extremities  of  the  roots,  it  immediately 
fills  the  cells  and  vessels  of  the  plant,  and  becoming  subjected 


NATURE   AND    SOURCES    OP   FOOD.  97 

to  their  vital  action,  undergoes  a  change  of  properties.  The 
water  thus  altered  is  called  the  crude  or  ascending  sap.  This 
fluid,  in  the  active  periods  of  vegetation,  is  incessantly  in 
motion,  and  is  unquestionably  analogous  to  the  blood  of  ani- 
mals. But  the  motion  of  the  sap  in  plants  is  a  great  deal  more 
complicated  and  altogether  different  from  the  circulation  of  the 
blood  in  animals.  The  sap  is  not,  like  the  blood,  confined  to  a 
separate  system  of  vessels,  for  owing  to  the  manner  in  which 
the  vascular  and  cellular  tissues  are  interwoven  with  each 
other,  and  the  general  permeability  of  all  the  organs,  a  general 
transfusion  of  the  sap  from  cell  to  cell  takes  place  endosmoti- 
cally  in  every  direction.  This  is  particularly  the  case  at  the 
commencement  of  growth,  as  in  germinating  plantlets,  or 
developing  leaf-buds,  but  as  soon  as  woody  fibre  and  vascular 
tissue  or  ducts  are  formed,  they  take  the  most  active  part  in 
the  upward  conveyance  of  the  sap  for  which  they  are  well 
adapted  by  their  tubular  and  capillary  character. 

The  current  of  ascending  sap  flows  through  the  vitally  active 
and  forming  cells  of  the  alburnum  or  sap-wood,  situated  nearest 
the  bark,  and  not  at  all  through  the  dead  wood  cells  of  the 
duramen  or  heart-wood,  situated  in  the  interior  of  the  stem. 
It  is  this  interposed  stratum  of  sap  which  renders  the  bark  and 
wood  so  easily  separable  in  the  spring  of  the  year. 

The  sap  in  plants  appears  to  be  set  in  motion  by  the  expan- 
sion of  the  buds.  '  The  extremities  of  the  branches  are  always 
more  herbaceous  than  the  part  of  the  branches  immediately 
below  them,  and  therefore  are  the  first  to  be  affected  by  an 
increase  of  temperature  in  early  spring.  So  soon  as  the  extre- 
mities of  the  branches  together  with  the  buds  begin  to  swell, 
the  cells  of  which  they  are  composed  attract  the  sap  from  the 
tissues  in  their  immediate  neighborhood,  which  tissues  are 

9* 


98  COMPOUND   ORGANS   OF   PLANTS. 

Fig.  14. 


Fig.  14.  Experiment  of  Hales  to  show  the  force  with  which  the  ?ap  ascends,  c.  Stock 
of  vine  cut.  t.  Tube  with  double  curvature  fastened  to  the  top  of  the  stock  by  a 
copper  cap  v,  which  is  secured  by  a  lute  and  piece  of  bladder  m.  n  n.  Leve'  of  the 
mercurial  column  in  the  two  branches  of  the  tube  at  the  commencement  of  the  experi- 
ment, n'  n'  Level  at  its  close . 


NATURE    AND    SOURCES   OF    FOOD.  99 

again  refilled  by  the  flow  of  the  sap  from  the  subjacent  tissues, 
and  in  this  manner  the  sap  is  gradually  set  in  motion  from  the 
extremities  of  the  branches  to  the  roots  through  the.  entire  sys- 
tem of  the  plant.  When  at  length  the  young  branches  have 
developed  themselves  from  the  buds,  and  the  leaves  are  spread 
abroad  in  the  atmosphere,  the  ascent  of  the  sap  becomes 
powerfully  accelerated  by  the  evaporation  which  takes  place 
from  their  surface. 

The  height  to  which  the  sap  rises  in  forest  trees  is  very 
great,  and  the  force  with  which  it  ascends  is  very  considerable. 
The  force  with  which  the  sap  ascends  in  the  stem  of  the  vine 
was  measured  by  Hales,  a  celebrated  English  physician.  In 
the  early  part  of  the  month  of  April,  he  fitted  a  bent  tube  to 
one  extremity  of  the  stem  of  a  grape-vine,  which  he  had  cut 
down  to  about  two  and  a  half  feet  above  the  ground.  This 
tube  was  graduated  and  its  curve  filled  with  mercery.  In  a 
few  days  he  found  that  the  ascending  force  of  the  sap  had  raised 
the  mercury  upwards  of  38  inches.  Now,  since  the  pressure 
of  the  atmosphere  supports  a  column  of  mercury  varying  from 
28  to  30  inches  in  height,  it  follows  that  the  ascending  force 
of  the  sap  is  greater  than  the  pressure  of  the  atmosphere.  In 
some  of  his  experiments,  Hales  calculated  that  the  ascending 
force  of  the  sap  in  the  stem  of  the  vine  was  five  times  greater 
than  that  which  impels  the  blood  through  the  principal  artery 
of  the  horse.  A  piece  of  bladder  tied  over  the  stuinp  of  another 
vine,  from  which  a  piece  had  been  cut  off  early  in  May,  was 
torn  into  shreds  by  the  rising  of  the  sap. 

As  the  sap  rising  in  the  stem  attains  a  greater  distance  from 
the  root,  it  becomes  less  watery  and  more  thick  and  mucilagi- 
nous. It  finds,  in  effect,  amassed  in  the  tissues  which  it  tra- 
verses, portions  of  gum,  sugar,  starch,  &c.,  left  in  them  by  the 


100  COMPOUND  ORGANS  OP  PLANTS. 

growth  of  the  previous  year,  which  it  re-dissolves  and  carries 
along  with  it ;  so  that  the  sap  which  circulates  in  the  superior 
parts  of  a  plant  offers  a  composition  more  rich  in  organic  prin- 
ciples. 

It  is,  however,  principally  in  the  leaves  that  the  sap  under- 
goes those  changes  which  render  it  subservient  to  the  growth 
and  nutrition  of  the  plant.  In  the  leaves,  the  sap  is  exposed 
to  the  influences  of  the  light  and  air,  and  is  thickened  and 
condensed  by  the  evaporation  of  the  useless  water.  Under  the 
influence  of  light,  the  oxygen  of  the  carbonic  acid  is  given  off 
from  the  leaves  into  the  atmosphere,  and  the  carbon  is  fixed, 
chlorophyl  being  formed  in  the  cells.  The  sap  is  distributed 
to  all  parts  of  the  leaf  by  means  of  the  veins  in  the  leaf, 
which  are  immediately  connected  with  the  alburnum  or  sap- 
wood  of  the  stem.  The  mechanism  of  the  leaf  and  the  action 
of  the  pores  has  been  already  explained.  Not  only  the  leaves, 
but  the  young  branches,  scales,  and  all  the  herbaceous  or  green 
parts  of  the  plant,  act  on  the  atmosphere  in  a  similar  manner 
to  the  leaves. 

After  having  been  elaborated  in  the  leaves,  the  sap,  which  is 
now  called  the  proper  juice,  re-descends  from  the  leaves  towards 
the  root. 

The  vascular  and  cellular  system  of  the  leaf  not  only  offers 
the  same  composition  as  the  stem,  but  it  preserves  the  same 
relative  situation  in  the  leaf  as  in  the  stem;  those  vessels 
which  occupied  the  interior  of  the  stem  next  to  the  pith, 
becoming  superior  in  the  leaf  whilst  the  more  external  vessels 
become  inferior,  and  all  retaining  the  same  relative  parallelism 
in  the  petiole  and  lamina. 

Now  the  fibro- vascular  tissue  which  thus  issues  from  the 
stem  into  the  petiole,  consists  of  two  layers  of  vessels,  an 


NATURE   AND    SOURCES   OF   FOOD.  101 

Fig.  15. 


Fig  15.  Vertical  section  through  a  young  branch  and  petiole,  showing  the  manner 
in  which  the  vascular  and  cellular  tissues  of  the  leaf  communicate  with  those  of  the 
stem,  m  pith  of  the  stem ;  fv  nbro-vascular  tissue  next  the  pith  passing  into  the 
petiole  which  is  articulated  to  the  axis  ;  pc,  ^wf parenchyma  of  the  stem ;  &,  bud  in  the 
axil  of  the  leaf;  c,  cushion  or  swelling  below  the  leaf;  /,  forming  fracture. 

ex-current  layer  situated  on  the  upper  surface  of  the  petiole 
and  lamina,  and  which  is  immediately  connected  with  the 
alburnum  of  the  stem,  and  a  recurrent  layer  situated  imme- 
diately beneath  the  first  layer,  on  the  under  surface  of  the 
petiole  and  lamina,  which  is  connected  with  the  endophleum  or 
inner  fibrous  bark.  The  sap  is  brought  from  the  albur- 
num by  the  ex-current  or  upper-layer,  into  the  leaf  and  distri- 
buted to  all  parts  of  its  upper  surface ;  having  undergone  all 
those  chemical  changes  which  render  it  suitable  for  vegetable 
assimilation,  or  having  been  elaborated  into  proper  juice, 
then  conveyed  by  the  recurrent  layer  of  fibres  along  the  under 
surface  of  the  lamina  and  petiole  into  the  bark,  down  which  it 
descends  to  the  roots. 

That  the  sap  re-descends  from  the  leaves  to  the  roots  by  the 


102          COMPOUND  ORGANS  OP  PLANTS. 

bark  is  evident  from  the  following  simple  experiment.  If  a 
ring^of  bark  be  removed  from  a  tree  in  spring,  the  sap  will 
rise  just  the  same  as  usual,  but  when  the  sap  begins  to  descend, 
a  protuberance  will  be  formed  just  above  the  ring,  which  is 
occasioned  by  the  accumulation  of  sap  there,  its  farther  descent 
being  stopped  by  the  removal  of  the  bark.  The  same  effect 
will  be  produced  if  we  make  a  simple  ligature  or  annular  com- 
pression of  a  young  stem.  At  the  end  of  a  year  or  two  a  cir- 
cular swelling  will  form  itself  immediately  above  the  ligature. 
This  swelling  is  evidently  produced  by  the  sap,  which  descending 
through  the  thickness  of  the  bark  from  the  summit  of  the 
stem,  an.d  finding  an  obstacle  which  it  cannot  pass,  accumulates 
•above  that  obstacle.  All  must  have  observed  the  distortions 
which  twining  stems  thus  produce  on  the  trunks  of  the  trees 
about  which  they  entwine  themselves. 

So  also  we  see  the  reason  why  the  branch  of  a  fruit  tree, 
when  sterile,  may  be  made  to  flower  and  fruit  abundantly  by 
being  girdled.  This  consists  in  removing  a  narrow  ring  of 
bark  from  the  branch,  sufficient  to  arrest  the  downward  course 
of  the  elaborated  sap,  which  is  thus  accumulated  in  the  branch 
sufficient  in  quantity  to  produce  this  desirable  result. 

The  ascending  and  descending  sap  are  very  different  both  in 
appearance  and  qualities.  The  ascending  sap  in  all  plants  is 
nearly  the  same,  containing  no  noxious  qualities  even  in  the 
most  poisonous.  We  are  told,  by  Berthellot,  that  the  natives 
of  the  Canary  Islands  tear  off  the  bark  from  the  poisonous 
Euphorbia  Canarensis,  and  find  the  ascending  sap  which  they 
obtain  from  the  alburnum  a  refreshing  drink,  whilst  the  des- 
cending sap  is  of  so  acrid  a  nature  that  it  acts  as  a  caustic, 
burning  the  flesh  off  such  as  happen  to  touch  it.  In  the  maple, 
and  some  other  plants,  the  ascending  sap  is  so  sweet  that  sugar 


NATURE   AND   SOURCES   OP   FOOD.  103 

may  be  obtained  from  it  by  evaporation,  whilst  the  descending 
sap  of  the  same  tree*  does  not  possess  any  sweetness. 

Besides  this  general  circulation  of  the  sap  through  the  entire 
cellular  and  vascular  system  of  the  plant,  an  independent  cir- 
culation or  movement  of  rotation  has  been  observed  in  the  cells 
themselves,  considered  separately  and  individually.  This  is 
well  seen  in  those  cells  which  form  the  hairs  of  plants  which 
are  conveniently  situated  for  observation.  The  string  of  bead- 
like  cells  which  compose  the  jointed  hairs  of  the  Tradescantia 
Virginica,  or  the  spiderwort,  show  this  circulation  distinctly 
under  a  magnifying  power  of  about  400  diameters.  In  the  tubu- 
lar cells  of  Chara,  an  aquatic  plant  growing  in  stagnant  pools, 
this  circulation  may  be  seen  with  an  ordinary  microscope.  The 
motion  of  the  currents  in  the  cells  of  these  plants  is  rendered 
visible  by  the  minute  grains  of  chlorophyl  which  they  carry 
along  with  them.  The  cause  of  these  motions  is  at  present 
wholly  unknown. 


PART    IV. 

ON    THE 

ORGANS    OF    REPRODUCTION 

IN  PHANEROGAMOUS  PLANTS. 


10 


PART     IV. 

THE  ORGANS  OF  REPRODUCTION  IN  PHANEROGAMOUS 
PLANTS. 


CHAPTER   VIII. 

GENERAL  CONSIDERATIONS  ON  THE  FLOWER. 

ALL,  intelligent  naturalists  are  agreed  that  a  similar  prin- 
ciple and  plan  of  structure  pervades  the  whole  chain  of  animal 
organization,  and  that  the  same  organs  are  developed  under 
different  forms  to  meet  the  peculiar  wants  and  self-preservative 
instincts  of  the  animal.  Thus,  the  arm  of  man,  the  foreleg  of 
quadrupeds,  the  true  wing  of  birds,  and  even  the  pectoral  fin 
of  fishes,  all  represent  one  and  the  same  organ  developed  under 
widely  different  forms,  in  accordance  with  those  purposes  to 
which  they  are  subservient  in  the  animal  economy. 

Now  precisely  the  same  is  the  principle  and  plan  of  struc- 
ture in  the  vegetable  world.  The  essential  organs  of  plants 
consist  of  the  root,  stem  and  leaves ;  no  new  organ  is  intro- 
duced, but  these  common  elements  of  vegetable  structure  are 
developed  in  peculiar  and  appropriate  forms  to  suit  the  several 
wants  of  the  plant. 

When  we  look  at  a  plant  in  full  bloom,  we  are  apt  to  regard 
it  as  an  organized  being  of  a  very  complex  character,  and  to 
look  on  the  green  leaves  of  its  stem,  and  the  several  members 


108          COMPOUND  ORGANS  OF  PLANTS. 

or  component  parts  of  its  flower,  as  entirely  distinct  in  their 
derivation  and  character.  A  more  extensive  acquaintance  with 
floral  structure  soon,  however,  discloses  the  interesting  and 
important  fact,  that  all  the  beautiful  and  highly  organized  parts 
of  the  flower  are  only  a  series  of  progressively  metamorphosed 
leaves ;  which  have  assumed  these  lovely  colors  and  this  pecu- 
liar arrangement  and  form,  in  consequence  of  the  peculiar 
functions  assigned  them. 

The  green  leaves  on  the  stem  and  branches  are  concerned  in 
the  functions  of  nutrition ;  they  decompose  carbonic  acid  gas, 
and,  under  the  influence  of  solar  light,  chlorophyl  is  formed  in 
their  cells,  (*?uo£o$  green,  and  QVM.OV  a  leaf,)  so  called  because 
it  is  the  substance  which  gives  to  the  leaves  their  green  hues. 
The  leaves  of  the  stem  take  their  peculiar  color  and  form  in 
consequence  of  their  action  on  the  atmosphere ;  they  take  in 
food  from  the  air,  which,  in  connection  with  that  absorbed  by 
the  roots  from  the  soil,  contributes  directly  to  the  growth  or 
the  extension  of  the  parts  of  the  plant. 

The  leaves  which  constitute  the  flower,  on  the  other  hand, 
are  concerned  in  the  functions  of  reproduction,  and  are  there- 
fore modified  in  their  structure,  form,  arrangement,  and  color, 
so  that  they  are  beautifully  adapted  to  the  exercise  of  these 
functions.  The  organs  of  reproduction  which  are  collectively 
designated  as  the  flower,  are  therefore  only  a  peculiar  modifica- 
tion of  the  organs  of  nutrition.  A  flower-bud  only  differs  from 
a  leaf-bud  in  having  no  power  of  extension.  Like  the  leaf-bud, 
it  is  a  shortened  branch,  the  axis  of  which  has  not  been  elon- 
gated, and  however  the  parts  of  the  flower  may  differ  from  the 
ordinary  leaves  of  the  plant  in  appearance,  we  shall  presently 
show  that  they  may  all  be  referred  to  the  leaf  as  a  type,  their 
nature  being  precisely  the  same,  and  appearance  dissimilar  in 
consequence  of  a  difference  in  the  functions  assigned  them. 


GENERAL   CONSIDERATIONS.  109 

Hence  when  the  student  has  acquired  a  knowledge  of  the 
anatomy  and  functions  of  leaves,  he  is  prepared  to  enter  on 
the  consideration  of  the  floral  organs. 

It  has  been  shown  that  every  plant  which  consists  of  more 
than  one  cell,  or  of  a  series  of  cells  united  together,  may  be 
divided  into  two  distinct  parts,  to  which  separate  functions  are 
assigned,  a  vegetative  part  and  a  reproductive  part.  In  the  more 
highly  organized  plants,  the  vegetative  part  of  the  plant  con- 
sists of  the  root,  the  stem,  and  the  leaves,  each  having  distinct 
functions  assigned  in  the  vegetable  economy.  Now  every 
plant  continues  to  grow  so  long  as  its  vegetative  cells  continue 
to  develop ;  but  when  the  plant  acquires  all  its  developments,  or 
arrives  at  an  adult  state,  the  reproductive  cells  show  them- 
selves, and  growth  stops  in  that  direction :  the  whole  force  of 
vegetation  being  expended  in  the  production  of  the  spore  or 
seed,  the  embryo  or  germ  of  the  future  plant. 

In  the  more  highly  organized  plants,  the  cells  which  are 
connected  with  reproduction  make  their  appearance  in  the 
form  of  beautiful  whorls  of  metamorphosed  and  colored  leaves, 
constituting  that  part  of  the  plant  which  is  popularly  called 
the  flower ;  and  we  are  about  to  trace  those  curious  processes 
which  are  carried  on  by  them,  or  their  physiological  action  in 
the  production  of  the  embryo  or  seed,  which  contains  within 
itself  the  rudiments  of  future  generations. 

That  flowering  is  an  exhaustive  process  and  therefore 
necessarily  causes  the  cessation  of  the  growth  or  extension  of 
the  parts  of  plants,  is  evident  from  the  following  facts.  Plants 
will  continue  to  grow  if  the  flower  buds  are  removed  as  soon 
as  they  are  formed.  This  is  often  done  by  gardeners,  who  nip 
off  the  young  flower  buds  in  order  to  encourage  the  growth  of 
the  plant,  which  is  thus  enabled  to  accumulate  a  greater  store 

10* 


110  COMPOUND   ORGANS   OF     PLANTS. 

of  nutriment,  and  finally  to  produce  finer  flowers  and  fruit. 
By  removing  their  flower  buds  as  soon  as  formed  and  thus 
preventing  the  exhaustion  consequent  on  flowering,  annuals 
may  be  changed  into  biennials,  or  even  perennials,  their  life 
being  prolonged  indefinitely;  whilst  the  same  plants  left  to 
flower  in  the  ordinary  course  of  nature,  perish  as  soon  as  they 
flower  and  bear  seed,  whether  during  the  first,  second,  or  any 
succeeding  year.  The  actual  consumption  of  nutriment  in 
flowering,  is  seen  iij  the  rapidity  with  which  the  farinaceous 
and  saccharine  store  accumulated  in  the  roots  of  the  beet  and 
carrot  disappears  as  soon  as  these  plants  begin  to  flower, 
leaving  them  light,  dry  and  empty ;  so  also  the  esculent  roots 
of  radishes  and  turnips  become  fibrous  and  unfit  for  food,  when 
they  are  allowed  to  run  to  seed.  When  the  branch  of  a  fruit 
tree  is  sterile,  it  may  be  made  to  flower  and  fruit  abundantly 
by  being  girdled.  This  consists  in  the  removal  of  a  narrow 
ring  of  bark  from  the  branch,  sufficient  to  arrest  the  downward 
course  of  the  elaborated  sap,  which  is  thus  accumulated  in  the 
branch  in  a  sufficient  quantity  to  produce  this  desirable  result. 

The  reproductive  organs  show  themselves  only  at  the  epoch 
when  the  plant  acquires  all  its  development,  or  arrives  at  an 
adult  state.  The  period  when  this  event  occurs  depends  on  the 
peculiar  organization  of  the  plant.  At  this  time  a  change 
takes  place  in  the  primary  mode  of  development,  the  buds  in 
the  axils  of  the  leaves  or  at  the  extremities  of  the  branches 
cease  to  elongate,  and  the  internodes  or  naked  intervals  of  stem 
between  the  leaves  being  non-developed,  the  leaves  remain 
crowded  together  in  whorls,  in  a  sort  of  rosette,  and  under- 
going peculiar  modifications  in  their  form  and  color,  a  flower  is 
produced. 

Every  flower,  when  complete,  consists  of  four  whorls  of  pro- 


GENERAL   CONSIDERATIONS.  Ill 

gressively  metamorphosed  leaves,  called  respectively  the  calyx, 
the  corolla,  the  stamens,  and  pistils.  Of  these  four  verticils, 
the  two  outer  whorls  marked  a  and  b,  in  Fig.  16,  are  called 


floral  envelopes,  and  are  considered  to  be  merely  accessary 
organs,  whose  functions  are  to  protect  the  two  inner  whorls, 
the  stamens  and  pistils  marked  c  and  d,  which  are  named 
sexual  organs,  and  which  are  by  far  the  most  important  and 
highly  organized  parts  of  the  flower.  A  flower  may  be  perfect 
and  reproduce  itself  without  either  calyx  or  corolla,  but  not 
without  stamens  or  pistils;  for  these  last  organs  are  imme- 
diately connected  with  the  formation  of  the  seed,  the  germ  of 
the  future  plant,  and  without  these  secreting  and  all-important 
bodies,  it  is  impossible  for  fertilization  to  take  place,  or  seed  to 
be  produced. 

The  leaves  of  the  flower,  like  those  of  the  stem,  are  arranged 
spirally  about  the  axis  of  growth,  and  therefore  the  separate 
pieces  of  each  verticil  alternate  with  each  other.  Thus  the 
petals  or  leaves  of  the  corolla  alternate  with  the  sepals  or  leaves 
of  the  calyx ;  that  is  to  say,  each  petal  is  placed  in  the  inter- 
val between  two  sepals ;  the  stamens  alternate  with  the  petals 
and  the  pistils  with  the  stamens. 

The  sepals  of  the  calyx,  or  outermost  of  the  floral  whorls, 


112  COMPOUND  ORGANS  OP  PLANTS. 

are  usually  colored  green,  and  are  the  nearest  allied  to  the 
leaves  of  the  stem,  both  in  form  and  appearance ;  the  petals  of 
the  corolla,  or  innermost  floral  envelope,  are  usually  of  some 
other  color  than  green,  as  for  instance,  white,  red,  blue,  yellow, 
or  some  intermediate  shade  of  these  colors,  and  more  delicate 
and  beautiful  in  their  texture  than  fflie  sepals.  The  stamens 
marked  c  in  fig.  16,  are  situated  immediately  within  the  corolla, 
and  surround  the  pistils  marked  d,  or  central  organs  of  the 
flower.  The  stamens  are  collectively  termed  the  androecium 
(avqp  a  male,  and  oixlov  habitation),  and  are  considered  to  be 
the  male  organs  ^  the  plant.  The  pistils  occupy  the  centre 
of  the  flower,  are  surrounded  by  the  stamens  and  floral 
envelopes,  and  after  flowering  are  changed  into  fruit,  and  con- 
tain the  seed.  The  pistils  are  collectively  termed  the  gyninoe- 
cium  (ywri  a  female,  and  faxiw  a  habitation),  and  are  considered 
to  be  the  female  organs  of  the  plant. 

All  these  organs  of  the  flower  are  situated  on  the  summit 
of  the  peduncle  or  flower-stalk,  and  the  part  on  which  they 
are  situated  has  received  the  name  of  thalamus,  torus,  or 
receptacle. 

The  different  organs  of  the  flower  are  verticillate  leaves 
brought  into  close  proximity,  in  consequence  of  the  non-devel- 
opment of  the  floral  internodes.  This  fact  is  beautifully  con- 
firmed by  the  appearance  of  an  internode,  or  naked  portion  of 
stem,  in  some  species  between  one  or  more  of  the  floral  whorls, 
by  which  they  become  separated  from  each  other,  just  as  the 
whorls  of  leaves  are  separated  on  the  stem.  Thus  the  inter- 
node,  or  naked  interval  of  stem  between  the  stamens  and  pistils 
is  developed  in  Euphorbia  corollata,  flowering  spurge  (Fig. 
17),  the  pistil  a  being  elevated,  after  it  is  fertilized,  on  a  little 
stalk,  and  thus  lifted,  as  it  were,  from  out  of  the  midst  of  the 


GENERAL   CONSIDERATIONS. 


113 


stamens  and  floral  envelopes ;  so  also  in  the  genus  Gynandrop- 
sis  (Fig.  18),  which  belongs  to  the  caper  family,  the  stami- 

Fig.   18. 

V     \<S^      d 
Fig.  17. 


nate  leaves  marked  s  are  separated  from  those  of  the  corolla  c, 
by  the  development  of  the  internode  or  naked  interval  of  stem 
between  them ;  and  the  pistil  p  is  also  separated  from  the  sta- 
mens by  the  development  of  another  internode,  and  supported, 
as  it  were,  on  a  little  stalk  or  pedicel.  Usually,  however,  the 
floral  internodes  remain  undeveloped,  and  therefore  such 
appearances  of  the  whorls  may  be  justly  regarded  as  an  ab- 
normal condition  of  things.  Aberrant  forms  .and  monstrosi- 
ties, whether  in  the  vegetable  or  in  the  animal  world,  are 
always  exceedingly  instructive,  and  furnish  rich  materials 
towards  cultivating  and  expanding  our  knowledge  of  the  regu- 
larly developing  organism. 


114 


COMPOUND  ORGANS  OF  PLANTS. 


CHAPTER    IX. 

THE   INFLORESCENCE. 

THE  arrangement  of  flowers  on  the  stem  or  floral  axis  is 
called  the  inflorescence.  Flower  buds,  like  leaf  buds,  are  either 
terminal  or  lateral.  Flowers  are  terminal  when  the  bud 
which  terminates  the  axis  of  growth  is  a  flower-bud.  This  of 
course  stops  the  farther  growth  of  the  plant  in  that  direction. 
Flowers  are  lateral  when  the  bud  which  terminates  the  axis  of 
growth  develops  as  a  leaf-bud.  In  this  case  the  floral  axis 
goes  on  extending  itself  indefinitely,  and  the  flowers  spring 
from  the  sides  of  the  axis  of  growth,  as  shown  in  Fig.  19, 
from  the  axilla  marked  b. 

Fig.  19. 


The  Bracts,  or  floral  leaves.  These  bracts  are  situated  all 
along  the  floral  axis  at  the  basis  of  the  peduncle  or  flower- 
stalk,  and  are  simply  the  ordinary  leaves  of  the  stem  reduced 
in  size  in  consequence  of  the  absorption  of  nutriment  from 
them  by  the  flower.  These  bracts  become  smaller  in  propor- 


THE   INFLORESCENCE.  115 

tion  as  they  approach  the  upper  part  of  the  floral  axis.  Hence 
the  leaf  gradually  passes  into  the  bract  in  consequence  of  its 
development  in  the  neighborhood  of  the  flower,  and  the  same 
proximity  doubtless  produces  the  abortive  leaves  of  the  calyx. 

Sometimes,  however,  the  bracts  are  as  richly  colored  as  the 
petals  themselves,  as  in  Castilleja  euchroma,  or  the  painted 
cup,  which  owes  all  its  beauty  to  its  conspicuous  and  deep 
scarlet  bracts.  The  curious  envelope  of  the  Indian  turnip, 
(Arum  triphyllum),  and  the  Ethiopian  lily,  (Calla  Ethiopica), 
called  a  spathe,  is  nothing  but  a  colored  bract ;  so  also  the 
conspicuous  petal-like  involucre  or  bract  of  the  dogwood, 
(Cornus  florida),  is  much  more  showy  than  the  real  flowers 
which  it  surrounds. 

Bracts  are  generally  distinct  from  each  other ;  but  when  the 
flowers  are  brought  together  and  situated  on  a  common  recep- 
tacle as  in  Umbelliferous  and  Composite  plants,  the  bracts  are 
also  brought  together  and  surround  the  basis  of  the  general 
receptacle  in  one  or  more  verticils  or  whorls.  In  the  Umbelli- 
ferse,  there  is  usually  a  whorl  of  bracts  surrounding  the  general 
umbel,  which  is  called  an  involucre,  and  in  some  genera  another 
whorl  of  bracts  also  surrounds  the  umbellets,  termed  an 
involucel.  In  the  Compositse,  the  involucre  consists  of  several 
rows  of  imbricated  bracts  which  surround  the  head  of  flowers, 
as  in  the  Aster,  the  Solidago  and  the  Helianthemum.  Not 
unfrequently  the  separate  flowers  also  are  subtended  by  bracts, 
termed  palese  or-  chaff.  In  the  grasses,  bracts  occupy  the  place 
of  both  calyx  and  corolla.  They  form  the  cupula  or  cup  of 
the  acorn,  and  also  the  husky  covering  of  the  hazel-nut. 

The  leaf  appears  to  pass  by  means  of  the  bract  into  the 
sepal  or  calyx  leaf.  There  is  in  reality  no  exact  limits  between 
common  leaves  and  bracts,  and  the  limits  between  bracts  and 


116  COMPOUND   ORGANS   OF    PLANTS. 

sepals  are  equally  imperceptible,  such  is  the  gradual  transition 
of  one  into  the  other.  The  gradual  transition  of  the  bract 
into  the  sepal  is  well  seen  in  composite  flowers  such  as  the 
marigold,  the  involucre  or  calyx  of  which  is  composed  of 
numerous  bracts  and  sepals  more  or  less  soldered  together. 
The  same  transition  is  also  visible  in  the  common  hollyhock  of 
the  gardens,  the  leaves  of  which  approximate  together,  become 
modified  in  size  and  appearance,  and  slide  as  it  were  insensibly 
into  a  calyx. 

As  flower  buds  are  produced  in  the  axils  of  bracts,  and  as 
bracts  are  only  modified  leaves,  it  follows  that  the  arrange- 
ment of  flower  buds  follows  the  same  law  as  the  arrangement 
of  leaf  buds,  the  flower  bud  being  merely  the  last  term  of 
ramification. 

When  the  flower  buds  are  lateral  and  the  inflorescence 
axillary  the  axis  elongates  indefinitely,  and  only  ceases  when 
the  terminal  bud  is  suppressed  or  on  the  approach  of  winter. 
When  the  floral  axis  elongates  in  this  manner,  the  lower 
flowers  are  the  first  to  expand,  whilst  those  towards  its  apex 
remain  closed,  and  the  expansion  is  said  to  be  centripetal  or 
from  the  circumference  to  the  centre.  For  when  a  floral  axis, 
developing  indefinitely,  is  shortened  by  the  non-development 
of  the  floral  internodes,  so  that  the  flowers  are  brought  together 
in  clusters  at  its  summit ;  the  outermost  flowers,  which  corres- 
pond to  the  lowest  flowers  of  the  lengthened  axis,  will  be  the 
first  to  expand,  whilst  the  innermost  flowers,  which  answer  to 
those- at  its  apex,  will  remain  closed.  The  expansion  of  the 
flowers  will  be  therefore  necessarily  centripetal,  or  from  the 
circurnferenoe  to  the  centre. 

When  the  flower  buds  are  terminal,  the  elongation  of  the 
floral  axis  is  necessarily  arrested ;  it  is  nevertheless  able  to 


THE   INFLORESCENCE. 


117 


extend  itself  by  secondary  and  tertiary  axes,  which  are  also 
arrested  in  their  growth  by  the  expanding  flower  at  their 
summit. 

If  we   take,  for   example,  the   inflorescence   of  Erythrsea 
centaurium,  (Fig.  20,)  we  shall   see  at  the   summit   of  the 

Fig.  20. 


primary  axis  a  flower,  a,  which  is  truly  terminal ;  but  from 
either  axis  of  the  first  pair  of  leaves  or  bracts  at  Z>,  arises  a 
secondary  axis,  each  axis  being  similarly  terminated  by  a 
single  flower,  and  bearing  also  two  pairs  of  bracts,  c,  c,  which 
in  their  turn,  give  rise  to  unifloral  tertiary  axes,  and  so  on. 

As  the  secondary  axis  arises  from  leaves  below  the  primary 
and  central  flower,  the  flower  at  the  apex  of  the  secondary 
3xis,  is  consequently  farther  removed  from  the  centre  of 
growth ;  and  the  same  remark  applies  to  the  flower  at  the 

11 


118        •  COMPOUND  ORGANS  OF  PLANTS. 

summit  of  the  tertiary  or  any  other  succeeding  axis  which 
may  be  developed.  In  this  case,  therefore,  since  the  flower 
terminating  the  growth  of  the  primary  axis  is  the  oldest, 
and  consequently  the  first  to  expand,  the  other  flowers 
expanding  in  succession  in  proportion  as  they  are  removed 
farther  from  the  centre, — the  expansion  of  the  flowers  is 
necessarily  centrifugal,  or  from  the  centre  to  the  circum- 
ference. 

This  mode  of  inflorescence  is  termed  a  cyme,  and  as  the 
divisions  in  this  case  always  take  place  by  two,  it  is  called  a 
dichotomous  cyme,  ($t*a,  two  ways,  and  -tip,™,  I  cut.)  If, 
instead  of  two,  three  whorled  leaves  or  bracts  developed  floral 
axes  in  a  similar  manner,  the  cyme  would  be  trichotomous, 
(fpt'^a,  in  three  ways.) 

The  inflorescence  has  received  different  names  according  to 
the  different  modes  in  which  the  flowers  are  arranged  on  the 
axis,  and  the  extent  to  which  that  axis  is  developed. 

The  following  are  the  leading  forms  assumed  by  the  indefi- 
nite or  indeterminate  inflorescence.  If  the  axillary  flowers  are 
without  a  peduncle  or  flower  stalk  and  sessile  along  the 
common  axis,  they  form  a  spike,  as  in  the  Plantain,  (Fig. 
21.)  If,  on  the  contrary,  the  axillary  flowers  are  supported 
on  a  peduncle  under  the  same  circumstances,  they  form  a 
raceme,  as  in  the  wild  cherry,  (Fig.  22.) 

If  the  floral  axis  of  a  spike  is  shortened  by  the  non-develop- 
ment of  the  floral  internodes,  a  capitulum  or  head  is  produced, 
as  in  *  Cephalanthus  occidentalis,  (Fig.  23.)  Sometimes  the 
capitulum  becomes  partially  elongated  into  a  spike  as  it  grows 
older,  as  in  Sanguisorba  and  many  species  of  Clover.  The 
shortened  axis  of  a  head  is  called  a  receptacle.  '  ^ 

Frequently,  instead  of  being  globular  as  in  Cephalanthus. 


THE    INFLORESCENCE. 


119 


Fig.  21. 


Fig.  22. 


Fig.  23. 


or  prolonged  as  in  Clover,  the  apex  of  the  floral  axis  is  dilated 
horizontally,  so  as  to  allow  a  large  number  of  flowers  to  grow 


120 


COMPOUND  ORGANS  OP  PLANTS. 


together  on  its  flat  or  convex  surface.  What  are  called  com- 
pound flowers,  as  the  Helianthemum,  Aster  and  Dandelion,  are 
heads  of  this  nature,  surrounded  by  a  common  involucre  of  bracts. 
This  flat,  dilated  receptacle,  is  very  conspicuous  in  the  Dande- 
lion after  its  ripe  pericarps  have  been  removed  by  the  wind. 

If  the  spike  be  succulent  and  covered  with  unisexual  flowers, 
ordinarily  incomplete,  that  is  to  say,  without  floral  envelopes, 
and  if  the  whole  be  enclosed  in  a  spathe,  the  inflorescence  is 
called  a  spadix,  as  in  Arum  maculatum,  (Fig.  24.) 

If  the  spike  be  covered  with  unisexual  flowers,  male  or 
female,  borne  in  the  axils  of  bracts,  the  axis  of  the  spike  being 
articulated  at  its  base  so  that  it  is  detached  and  falls  off  all  in 
one  piece,  the  inflorescence  is  termed  an  amentum  or  catkin, 
(Fig.  25.) 

Fig.  24.  Fig.  25. 


a 


"Fig.  25.  a  Unisexual  amentum  of  the  hornboam  (Oarpinus  bctulus.)    b.  One  of  tie 
'flowers  with  its  subtending  bract  magnified. 


THE    INFLORESCENCE. 


121 


When  the  floral  axis  of  a  raceme  is  so  shortened,  and  the 
peduncles  of  the  lower  flowers  are  so  elongated,  as  to  elevate 
them  to  the  same  level  as  the  upper  flowers,  a  corymb  is 
formed,  as  in  Achillea  millefolium.  If  the  floral  axis  of  a 
raceme  be  suppressed  altogether,  so  that  the  peduncles  all 
start  from  the  same  point,  we  have  an  umbel,  (Fig.  26.) 

Fig.  26.  Fig.  27. 


b  c 

Tig.  28.  Diagrams  of  a  corymb  b,  and  of  an  umbel  c. 


If  the  secondary  floral  axis  of  a  raceme  gives  rise  to  tertiary 
ones,  the  raceme  is  branching,  and  forms  a  panicle,  (Fig.  27.) 
The  panicle  ordinarily  assumes  a  pyramidal  form,  that  is  to  say, 
the  floral  axes  become  shorter  in  proportion  as  they  approach  the 
summit.  If  on  the  contrary,  the  floral  axes  of  the  middle  part 
are  the  longest,  the  inflorescence  takes  a  more  or  less  ovoid 
form,  and  is  denominated  a  thyrsus,  as  in  the  lilac. 

The  definite  and  determinate  inflorescence.  The  lowest 
developments  of  this  form  of  inflorescence,  is  that  in  which  a 
single  floral  axis  is  terminated  by  a  solitary  flower,  of  which 
the  Anemone  nemorosa  furnishes  a  good  example.  When  such 
an  inflorescence  branches,  the  branches  do  not  grow  in  an 

11* 


122  COMPOUND   ORGANS   OF    PLANTS. 

indeterminate  manner,  but  are  arrested  in  their  development 
by  the  terminal  flowers. 

The  most  common  and  regular  cases  of  determinate  inflo- 
rescence occur  in  opposite-leaved  plants.  In  these  plants  the 
inflorescence  is  composed  of  a  superposed  series  of  bifurcations 
of  the  primary  axis,  in  the  centre  of  each  of  which  a  terminal 
flower  is  situated.  This  mode  of  inflorescence,  which  is  termed 
a  cyme,  has  been  already  explained,  and  may  be  studied  to 
advantage  in  the  chickweeds,  (Cerastium  and  Stellaria,)  in 
which  it  is  recognizable  at  once  by  the  solitary  flower,  destitute 
of  bracteoles,  in  each  fork  of  the  branches. 

Fig.  28.  Fig.  29. 


Sometimes  only  one  of  the  two  bracts  on  the  primary  and 
succeeding  axes  developes  a  flower,  as  in  Arenaria  stricta,  (Fig. 
28 ;)  or  the  floral  bract  is  suppressed  altogether,  so  that  the 
flower  appears  opposite  the  remaining  bracts,  (Fig.  29 ;)  or 
both  bracts  are  suppressed,  and  the  flowers  only  are  developed, 
as  in  Myosotis  palustris,  (Fig.  30.)  When  this  is  the  case, 
the  cyme  assumes  a  remarkable  curvature,  turning  round  in  a 
paculiar  way  so  as  to  resemble  a  snail  or  the  tail  of  a  scorpion, 
and  hence  it  is  called  a  helicoid  or  scorpioid  cyme;  (i'jut,  a 
spiral,  and  n5oj,  fonr.)  This  form  of  inflorescence  may  be 


THE    INFLORESCENCE. 

"    Fig.  30. 


123 


observed  in  the  Heliotropium  Peruvianum,  in  the  Sedums,  in 
the  Droseras  or  sundews,  and  in  most  Boraginaceous  plants. 
The  theoretical  formation  of  this  inflorescence  may  be  ascer- 
tained by  consulting  the  ideal  figure  placed  here  by  the  side  of 
the  scorpioid  cyme  of  Myosotis  palustris. 

The  first  flower  is  situated  at  b,  and  terminates  the  growth  of 
the  primary  axis  a,  b  ;  from  the  axil  of  the  bract,  or  in  the 
place  where  it  is  suppressed  at  c,  arises  a  secondary  axis,  c,  d, 
which  by  its  vigorous  development,  usurps  the  place  of  the 
primary  axis,  which  is  thus  cast  to  one  side.  In  like  manner, 
a  tertiary  axis,  e,  f,  springs  from  the  axis  c,  d,  at  e,  the  ter- 
minal flower  at  d  becoming  apparently  lateral,  as  before ;  in 
this  manner  a  succession  o£  unifloral  axes  are  produced  from 


124          COMPOUND  ORGANS  OP  PLANTS. 

each  other,  which  have  the  appearance  of  a  continuous  primary 
axis ;  but  the  flowers  which  appear  lateral  are  in  reality  all 
terminal. 

As  might  be  expected,  all  these  forms  of  inflorescence  pass 
into  each  through  endless  intermediate  gradations.  In  nature 
they  are  not  so  absolutely  fixed  as  in  our  written  definitions, 
and  whether  this  or  that  name  should  be  used  in  a  particular 
case,  is  often  a  matter  of  fancy. 

The  manner  in  which  the  leaves  of  the  flower  are  arranged 
in  the  bud,  before  the  expansion  of  the  flower,  is  called  their 
aestivation,  (sestivus  belonging  to  summer,)  or  prsefloration, 
(pree,  before,  and  flos,  flower.)  These  terms  bear  the  same 
relation  to  the  flower  bud  that  vernation  does  to  the  leaf  bud ; 
and  indeed,  since  the  flower  bud  is  only  a  modified  leaf  bud,  as 
might  be  expected,  the  corresponding  terms  applied  to  vernation 
are  used  in  reference  to  praefloration  or  aestivation.  A  few 
new  terms  are  however  added,  descriptive  of  certain  peculiar 
modifications  in  the  general  forms  described  in  vernation. 
Fig.  31.  Fig.  32. 


Fig.  32.    The  flower  bud  of  Althsea  rosea,  showing  the  valves  of  the  calyx,  c,  opened, 
and  the  contorted  praefloration  of  the  petals  of  the  corolla,  p. 

The  following  are  the  principal  forms  of  aestivation  to  which 
the  others  are  generally  reducible.* 


THE   FLORAL   ENVELOPES.  125 

1.  The  valvate.     When   the   sepals   or   petals  fit  by  their 
edges,  without   overlapping   each   other,   as   in   the   mallow. 

2.  The  imbricated.     When  the  petals  or  sepals  cover  each 
other  by  a  part  of  their  height  merely,  like  the  tiles  of  a  roof. 
The  calyx  of  the  Camelia  japonica,  (Fig.  31,)  is  a  good  illus- 
tration of  the  imbricated  prsefloration. 

3.  The   contorted.  ,  When    the  petals  or  sepals  exhibit   a 
tortion   of  their  axis,  and  overlap  each  other's  margins,  the 
whole  appearing  to  be  more  or  less  spirally  twisted,  as  in  the 
flowers  of  the  Althaea  rosea,  (Fig.  31.) 


CHAPTER    X. 

t 

THE   FLORAL   ENVELOPES. 

IN  a  complete  flower  we  find,  without  the  stamens  and  pistils, 
two  whorls  of  progressively  metamorphosed  leaves,  the  calyx, 
which  is  the  exterior  whorl,  and  the  corolla  placed  immedi- 
ately within  the  calyx.  The  modified  leaves  of  the  flower  are 
brought  into  close  proximity  by  the  non-development  of  the 
floral  internodes,  in  order  that  the  several  whorls  may  the 
more  readily  communicate  with  each  other ;  which  immediate 
communication  is  necessary  to  the  production  of  the  seed. 
Let  us  now  examine  more  particularly  the  two  outermost 
whorls  of  floral  leaves,  designated  as  the  calyx  and  corolla. 

The  calyx,  so  named  from  xaxvt  a  cup.  This  forms  the 
outermost  whorl  of  the  floral  leaves,  and  consists  in  its  usual 
state  of  a  leafy  green  cup  more  or  less  divided.  The  sepals  or 
leaves  of  the  calyx  differ  but  slightly  in  structure  and  appear- 
ance from  the  ordinary  leaves  of  the  stem ;  they  are  for  the 


126  COMPOUND  ORGANS  OP  PLANTS. 

most  part  of  a  greenish  hue,  chlorophyl  being  formed  in  their 
cells,  and  stomata  or  pores  existing  on  their  lower  epidermis ; 
and  in  some  cases  of  monstrosity,  they  are  actually  converted 
into  the  ordinary  leaves  of  the  plant.  In  proliferous  states  of 
the  rose,  the  calyx  assumes  a  leafy  aspect ;  whilst  in  Gentiana 
campestris  and  Gentiana  crinita,  it  differs  in  no  respect  from 
the  ordinary  leaves  of  the  plant. 

The  sepals  of  the  calyx  are  sometimes  separate  from  each 
other  as  in  the  buttercup,  at  other  times  they  are  united  to  a 
greater  or  less  extent,  as  in  the  Polyanthus.  When  the  sepals 
are  separate  from  each  other,  whatever  may  be  their  number  the 
calyx  is  polysepalous  (jtofos  many,  sepala  leaves;)  but  the  term, 
as  currently  understood  amongst  botanists,  is  simply  used  to 
express  the  absence  of  cohesion  amongst  them,  and  is  equiva- 
lent in  meaning,  to  the  expression  sepals  distinct.  When  the 
sepals  of  the  calyx  are  united  to  each  other  by  their  margins 
in  a  greater  or  less  degree,  the  calyx  is  monosepalous  (^01/05 
one,  sepala  leaf.)  The  same  remarks  apply  to  the  petals  of  the 
corolla,  which  are  polypetalous  or  monopetalous,  according  as 
the  petals  are  separate  from  each  or  in  a  state  of  cohesion. 

It  is  well  known  that  the  parts  of  plants  which  grow  closely 
together  are  apt  to  cohere,  the  parts  anastomosing  with  each 
other.  Accidental  unions  of  this  kind  among  the  leaves  of 
plants  are  of  frequent  occurrence.  Now  owing  to  the  non- 
development  of  the  floral  internodes,  the  metamorphosed  leaves 
which  constitute  the  flower  are  necessarily  brought  into  closer 
contact,  and  hence  they  are  more  frequently  found  united  with 
each  other  than  the  leaves  of  the  stem. 

The  sepals  of  a  monosepalous  calyx  may  cohere  together 
by  their  bases,  or  by  their  margins,  through  their  inferior  half, 
or  through  their  entire  length,  and  various  terms  are  employed 
to  express  these  different  degrees  of  cohesion. 


THE   FLORAL   ENVELOPES.  127 

Fig.  33. 


I 

Fig.  33.  Thus  when  the  sepals  are  coherent  by  their  bases  as 
in  the  Pimpernel  a,  we  employ  the  terms  bi-partite,  tri-partite, 
quadri-partite,  according  as  there  are  two,  three,  or  four  sepals 
thus  united.  When  the  union  of  the  sepals  takes  place  through 
the  lower  half  Nof  their  margins,  such  sepals  are  bi-fid,  tri-fid, 
quadri-fid,  as  in  Erythrsea,  b.  If  the  sepals  are  united  with 
each  other  by  their  margins  nearly  to  their  summit,  they  are 
bi-dentate,  tri-dentate,  as  in  Lychnis,  c.  Finally,  if  the  union  of 
the  margins  is  complete  through  their  entire  length,  the  calyx 
is  said  to  be  entire.  It  is  seldom,  however,  that  the  cohesion  of 
the  calycine  leaves  is  complete,  and  the  number  of  lobes  at  the 
summit  of  the  calyx  will  in  general  show  the  number  of  sepals 
which  have  cohered  together.  In  the  entire  monosepalous 
calyx,  the  venation  assists  in  determining  the  number  of 
cohering  sepals. 

When  the  sepals  are  unequally  developed  and  united,  the 
calyx  is  said  to  be  irregular.  This  takes  place  in  the  Labiatae, 
or  mint  tribe,  where  some  of  the  sepals  of  the  calyx  unite  to  a 
greater  extent  than  others,  thus  forming  a  bi-labiate  or  two- 
lipped  calyx,  as  in  the  dead  nettle,  Lamium.  (Fig.  34.)  The 
upper  lip  is  composed  of  three  sepals,  the  lower  of  two ;  the 
united  parts  form  the  tube;  the  free  portions  the  lobes  or 
segments  of  the  limb;  and  the  part  where  they  join  one 
another  the  mouth  or  throat. 


128          COMPOUND  ORGANS  OF  PLANTS. 
Fig.  34.  ,    Fig.  35. 


The  monopetalous  corolla,  has  corresponding  terms  applied  to 
its  modifications  and  to  thadegrees  of  cohesion  amongst  its  petals. 

When  the  calyx  falls  as  soon  as  the  corolla  expands  it  is 
termed  caducous,  as  in  Sanguinaria  Canadensis,  which  is  at 
first  enclosed  in  a  calyx  of  two  leaves,  which  fall  off  as  soon  as 
the  flower  is  fully  blown.  The  calyx  is  deciduous  when  it 
drops  off  with  the  corolla,  but  in  many  cases,  the  calyx 
remains  after  the  corolla  and  other  floral  whorls  have  faded 
and  fallen,  as  a  protecting  envelope  to  the  fruit,  as  in  the 
mallow,  (Fig.  35.)  In  this  case  it  is  said  to  be  persistent  (per 
through,  and  sisto  to  remain.) 

Sometimes  the  calyx  and  fruit  cohere  together,  so  that  the 
calyx  appears  to  arise  from  the  summit  of  the  fruit,  as  in  the 
rose ;  such  a  calyx  is  called  a  superior  calyx ;  if,  on  the  con- 
trary, the  calyx  and  fruit  do  not  cohere  together,  the  calyx  is 
said  to  be  inferior,  as  in  the  strawberry.  In  some  plants 
the  calyx  is  suppressed  altogether,  or  it  may  be  present  and 
reduced  to  a  mere  rim  or  border,  as  in  the  Umbelliferse ;  or  to 
a  pappus,  as  in  Composite. 

A  great  many  plants,  however,  have  only  one  floral  envelope 
exterior  to  the  stamens  and  pistils,  as  for  example,  the  hyacinth 
and  the  lily.  The  early  botanists  differed  amongst  themselves 
as  to  the  term  by  which  this  single  floral  envelope  ought  to  be 
distinguished  from  the  others.  Tournefort  and  Linnaeus  called 
it  the  calyx  when  it  was  green  and  bore  the  general  character 


THE   FLORAL  ENVELOPES.  129 

of  a  calyx,  and  gave  it  the  general  name  of  corolla  when  by  its 
color  and  the  delicacy  of  its  tissue  it  approximated  to  that 
organ.  But  this  distinction  is  utterly  worthless,  for  the  same 
organ  may  vary  in  color  without  changing  its  nature.  Thus, 
in  the  Fuchsia  or  lady's  ear  drop,  and  in  Salvia  splendens,  one 
of  the  Mexican  sages,  the  calyx  is  of  the  same  bright  scarlet 
color  as  the  corolla ;  and  in  the  white  water-lily  and  magnolia, 
the  sepals  gradually  approximate  in  color  to  the  petals.  Hence 
it  is  now  agreed  amongst  botanists  when  a  flower  has  but  one 
envelope  to  its  stamens  and  pistils,  to  consider  it  as  a  calyx, 
whatever  may  be  its  color  and  form. 

The  corolla,  from  "  corolla"  a  garland,  is  that  part  of  the 
flower  situated  immediately  within  the  calyx,  between  the  calyx 
and  stamens.  It  is  generally  the  most  showy  and  beautifully 
colored  of  all  the  floral  organs,  and  is  the  part  which  is  popularly 
called  the  flower.  Thus  the  red  leaves  of  the  rose,  the  yellow 
leaves  of  the  buttercup,  constitute  the  corolla  of  these  plants. 

The  divisions  of  the  corolla  are  called  petals  from  (rteta^ov  a 
leaf.)  If  these  petals  are  united  by  their  margins  so  as  to  form 
apparently  one  petal,  as  in  the  primrose  and  Campanula  the 
corolla  is  termed  monopetalous;  if,  on  the  contrary,  the  petals 
do  not  cohere  together,  but  grow  separately  and  distinctly  apart 
as  in  the  rose,  the  corolla  is  said  to  be  polypetalous.  When 
the  various  divisions  or  petals  of  the  corolla  are  alike  and  its 
incisions  uniform,  the  corolla  is  regular;  if  otherwise,  it  is 
irregular.  The  lower  part  of  a  monopetalous  corolla  is  called 
the  tube,  the  upper  and  expanded  portion  the  limb,  and  the 
part  where  the  two  are  connected  with  each  other  the  throat. 

The  sepals  of  the  polysepalous  calyx  are  usually  sessile 
leaves,  having  nothing  analogous  to  a  leaf  stalk  at  their  base ; 
but  it  is  otherwise  with  the  petals  of  the  polypetalous  corolla, 

12 


130          COMPOUND  ORGANS  OP  PLANTS. 

These,  although  sometimes  sessile,  as  in  the  rose  and  crowfoot, 
have  not  ^infrequently  their  base  tapering  into  a  narrow  stalk 
analogous  to  the  petiole  of  the  leaf,  which  is  called  an  unguis 
or  claw ;  whilst  their  upper  portion,  which  corresponds  to  the 
blade  of  the  leaf,  is  broader  and  more  expanded,  and  is  called 
the  lamina,  as  in  the  wall-flower.  (Fig.  36.)  Petals  organized 
in  this  manner  are  termed  unguiculate. 

Fig.  36. 


Pig.  36.  Cruciform  corolla  and  unguiculate  petal  of  the  wall-flower,  (Cheiranthus.) 

The  following  are  some  of  the  leading  forms  assumed  by  the 
regular  polypetalous  corollas.  The  rosaceous,  of  which  the  rose 
is  the  type,  have  spreading  petals  without  claws  or  with  very 
short  ones.  The  cruciform,  in  which  there  are  four  petals, 
usually  with  claws,  arranged  in  the  form  of  a  cross,  as  in  the 
wall-flower.  The  liliaceous,  in  which  the  petals,  six  in  number, 
gradually  taper  from  the  base  to  the  apex,  as  in  the  lily.  The 
caryophyllaceous,  where  the  petals  have  long,  narrow,  tapering 
claws,  which  are  enclosed  in  a  tubular  calyx,  as  in  the  pink. 

Irregularities  in  the  form  of  polypetalous  corollas  may  result 
from  the  unequal  development  of  the  petals,  as  in  the  violet ; 
but  these  are  not  sufficiently  marked  as  to  justify  the  appli- 
cation of  any  particular  term.  There  is,  however,  one  form 


THE   FLORAL   ENVELOPES.  131 

of  irregularity  amongst  polypetalous  corollas  which  usually 
receives  a  special  notice,  on  account  of  the  remarkably  anoma- 
lous development  of  its  petals,  and  because  it  is  characteristic 
of  an  extensive  natural  order  of  plants,  viz  :  the  papilionaceous 
corolla,  from  (papilio,  a  butterfly,)  of  which  the  pea-flower 
furnishes  a  good  example.  (Fig.  37.)  This  corolla  is  composed 

Fig.  37. 


of  five  unequal  and  dissimilar  petals.  One  larger  than  the 
rest,  a,  called  the  vexillum  or  standard,  which  is  usually 
folded  over  the  other  petals  in  aestivation ;  two  lateral  petals, 
b,  which  are  designated  as  the  alee  or  wings ;  and  two  inferior 
petals,  usually  completely  covered  by  the  alse,  and  their  lower 
margins  so  united  as  to  form  a  single  keel-like  piece,  called 
the  carina  or  keel,  c.  This  last  piece  embraces  the  essential 
organs,  the  stamens  and  pistils. 

The  following  leading  forms  may  be  distinguished  amongst 
the  regular  monopetalous  corollas.  The  campanulate,  or  bell- 
shapfed,  as  in  Campanula  rotundifolia,  (Fig.  38,  a,)  which  is 
without  a  tube,  and  which  enlarges  gradually  from  the  base  to 
the  apex.  The  infundibuliform  or  funnel-shaped,  as  in  the 
Convolvulus  purpureus,  or  morning-glory,  in  which  the  tube 
is  narrow  below  but  widely-expanded  towards  the  summit. 
The  hypocrateriform  or  salver-shaped,  as  in  the  Phlox  (Fig. 
38,  6,)  where  the  limb  spreads  out  at  right-angles  with  the 


132 


COMPOUND  ORGANS  OP  PLANTS. 

Fig.  38. 


more  or  less  elongated  tube  of  the  corolla.  The  rotate  or 
wheel-shaped,  as  in  the  Myosotis  palustris  or  forget-me-not, 
which  is  a  salver-shaped  corolla  without  a  tube,  or  with  a  very 
short  one.  The  tubular  or  tube-shaped,  as  in  the  Caprifolium 
or  honey-suckle,  where  the  limb  is  not  developed  and  the 
corolla  is  cylindrical  or  tubular  throughout  its  entire  length. 
The  urceolate  or  urn-shaped,  as  in  Vaccinium  macrocarpon,  the 
American  cranberry,  in  which  there  is  scarcely  any  limb,  and 
the  tube  is  narrowed  at  both  ends  and  expanded  in  the  middle. 
Irregularities  in  the  form  of  monopetalous  corollas  are 
produced  by  differences  in  the  degrees  of  cohesion  amongst  the 
petals.  The  principal  forms  of  irregular  monopetalous  corollas 
are  : — the  labiate  or  lipped,  (from  labium,  a  lip,)  (Fig.  89,  a,) 
in  which  the  tube  is  more  or  less  elongated,  the  throat  open 
and  dilated,  and  the  limb  divided  traversely  in  such  a  way  as 
to  produce  an  upper  and  lower  portion  called  the  labia  or  lips, 
with  a  hiatus  or  gap  between  them,  like  the  mouth  of  an  ani- 
mal. The  upper  lip  is  usually  composed  of  two  petals,  as  the 


THE    FLORA.L   ENVELOPES. 
Fig.  39.  Fig.  40. 


133 


little  notch  at  its  summit  proves;  the  lower,  of  three.  When  the 
two  lips  are  thus  gaping  and  the  throat  open,  the  corolla  is 
said  to  be  ringent,  as  in  Lamium  amplexicaule ;  but  when  the 
mouth  is  closed  by  the  approximation  of  the  two  lips,  by  an 
elevated  protuberance  of  the  lower  called  the  palate,  as  in  the 
snap-dragon  or  toad-flax  5,  the  corolla  is  designated  as  per- 
sonate or  masked.  When  a  tubular  corolla  is  split  down  on 
one  side  in  such  a  way  as  to  form  a  strap-shaped  process  with 
several  tooth-like  projections  at  its  apex,  it  becomes  ligulate 
(ligula,  a  little  tongue,)  or  strap-shaped.  (Fig.  40,  d.)  This 
kind  of  corolla  is  well  seen  in  composite  flowers  such  as  the 
dandelion,  in  which  all  the  flowers  forming  the  head  are 
ligulate.  In  the  Compositae  there  are  often  two  kinds  of  florets 
associated  in  the  same  head.  Thus  the  outer  florets  which  form 
the  white  ray  of  the  Ox-eye  daisy  (Leucanthemum)  are  ligulate, 
whilst  those  which  form  the  yellow  disk  are  tubular,  (c.) 

The  largest  flower  in  the  world  is  the  Bafflesia  Arnoldii, 
(Fig.  41,)  which  was   discovered  by  Sir    Thomas  Stamford 

12* 


134  COMPOUND  ORGANS  OF  PLANTS. 

Fig.  41. 


Raffles  in  the  forests  of  Sumatra,  in  the  year  1818,  growing  on 
the  stems  of  the  Cissus  augustifolia,  a  kind  of  climbing  plant  or 
grape-vine.  In  the  bud  state  this  flower  is  nearly  a  foot  in 
diameter,  and  when  fully  expanded,  nine'  fe'et  in  circumference 
and  three  feet  over  from  the  tip  of  one  petal  to  that  of  another. 
Its  substance  is  about  half  an  inch  thick,  and  the  whole  plant 
weighs  fifteen  pounds.  Its  color  is  light  orange  mottled  with 
yellowish-white,  and  like  other  parasites,  it  derives  its  nutri- 
ment from  the  tree  on  which  it  is  found.  A  few  other  species 
of  less  gigantic  size  have  been  discovered  in  the  other  islands  of 
the  Eastern  archipelago. 

Structurally,  the  petals  or  leaves  of  the  corolla  are  composed 
of  cellular  and  vascular  tissue,  the  latter  consisting  of  spiral 
vessels  and  delicate  tubes.  The  color  of  the  petals  is  produced 
by  the  refined  and  splendidly  colored  juices  elaborated  from  the 
sap  by  the  walls  of  the  cells  which  form  their  tissue  or  substance. 
This  fact  is  easily  verified  by  submitting  to  microscopic  exami- 
nation a  fragment  of  the  petal  of  a  rose  or  of  a  camellia,  when 
it  will  be  seen  that  the  color  does  not  exist  in  the  walls  of  the 
cells  of  the  petal,  but  it  is  the  result  of  the  colored  fluids  with 
which  the  cells  are  filled. 


THE    FLORAL  ENVELOPES.  135 

Sometimes,  by  the  mere  juxtaposition  of  the  different  cells 
in  the  petals,  a  mechanical  admixture  of  their  various  contents 
takes  place ;  thus  is  probably  produced  that  delicate  and  inimi- 
table shading  seen  in  the  petals  of  some  flowers;  at  other 
times,  the  petals  are  spotted  and  variegated,  as  in  the  tiger 
lily  and  balsam.  Such  spots  result  from  the  peculiar  power, 
possessed  by  some  of  the  cells,  of  attracting  from  the  colorless 
sap  these  particular  colors,  and  of  which  power  the  other  cells 
appear  to  be  deprived.  No  admixture  of  color  with  the  neigh- 
boring cells  takes  place  in  this  case.  "In  the  petals  of 
Impatiens  balsamina,  the  garden  balsam,"  says  Dr.  Lindley, 
"  a  single  cell  is  frequently  red  in  the  midst  of  others  that  are 
colorless.  Examine  the  red  bladder,  and  you  will  find  it  filled 
with  a  coloring  matter  of  which  the  rest  are  destitute." 

Every  one  must  have  noticed  the  regularity  with  which 
these  spots  are  formed  in  the  petals  of  certain  flowers,  which 
are  in  fact  never  without  them.  Such  cells  appear  to  have 
definite  functions  assigned  them,  the  exercise  of  which  is  pro- 
bably as  important  to  the  healthy  vital  action  of  the  plant  as 
that  of  the  most  elaborate  organs. 

The  chromule,  or  coloring  substance  of  plants,  is  by  no 
means  confined  to  their  petals,  but  sometimes  pervades  the 
sepals  of  the  calyx,  as  we  have  already  shown,  and  is  even 
occasionally  extended  into  the  tissue  of  the  bracts  and  ordinary 
leaves  of  the  stem.  The  beautiful  wild  flower  called  Castilleja 
euchroma,  or  the  painted  cup,  owes  all  its  beauty  to  its  con- 
spicuous and  deep  scarlet  bracts;  and  in  Croton  pictum,  a 
plant  which  may  be  frequently  met  with  in  conservatories,  the 
chromule  tints  the  ordinary  leaves  of  the  plant.  The  analogy 
of  the  petals  to  the  leaf  is  thus  clearly  traceable ;  and  how- 


136          COMPOUND  ORGANS  OF  PLANTS. 

ever  dissimilar  the  petals  of  the  corolla  may  appear  to  the 
ordinary  leaves  on  the  stem  of  some  plants,  so  that  we  may 
feel  disposed  to  regard  them  as  separate  organs,  yet  the 
evidence  afforded  by  these  transition  forms  shows  the  intimate 
connection  subsisting  between  the  petal,  the  sepal,  and  the 
bract,  and  the  common  origin  of  the  whole  of  them  from  the 
ordinary  stem-leaf,  of  which  they  are  but  modifications. 

Functions  of  the  Floral  Envelopes. — The  calyx  and  corolla 
are  by  far  the  most  conspicuous  and  showy  parts  of  the  flower, 
and  are  the  parts  of  it  which  usually  attract  popular  notice. 
Yet  their  functions  are  entirely  of  a  secondary  and  subordinate 
character.  The  internodes  between  the  several  whorls  of  floral 
leaves  are  not  developed,  in  order  that  they  may  the  more 
readily  act  on  each  other.  The  calyx  and  corolla  doubtless 
foster  and  protect  the  two  inner  whorls  of  leaves,  viz. :  the 
Stamens  and  pistils,  which  are  more  immediately  connected 
with  the  process  of  reproduction  than  they  are. 

All  must  have  noticed  the  folding  up  of  the  calyx  and 
corolla  at  sunset,  or  in  wet  weather.  The  function  exercised 
by  the  two  outer  whorls  of  the  floral  leaves  is  in  this  case 
clearly  protective,  and  the  design  of  their  close  proximity  to 
the  stamens  is  at  once  apparent ;  that  they  may  fold  over  the 
stamens  and  pistils,  and  thus  ward  off  the  injurious  effect  of 
the  night  dews  and  falling  rain,  which  would  act  injuriously 
on  the  pollen  contained  in  the  cells  of  the  anther.  Thus 
safely  and  beautifully  sheltered  at  every  epoch  of  their 
development,  the  stamens  and  pistils  perform  their  respective 
functions. 

The  bracts  and  calyx,  when  of  a  green  color,  doubtless  per- 
form the  same  functions  as  the  ordinary  leaves  of  the  stem ; 
but  it  is  otherwise  with  the  petals  of  the  corolla  and  with  the 


THE   FLORAL   ENVELOPES.  137 

other  parts  of  the  flower;  these  exercise  on  the  atmosphere, 
a  different  kind  of  influence.  Before  the  appearance  of 
the  flowers,  the  plant  is  wholly  an  apparatus  of  reduction, 
all  its  parts  being  concerned  in  the  assimilation  of  the  food. 
It  decomposes  the  carbonic  acid  borrowed  from  the  atmo- 
sphere and  the  soil,  fixing  the  carbon  and  exhaling  the  oxygen, 
and  forming  within  its  green  leaves,  young  shoots,  and  super- 
ficial parts,  the  substance  called  chlorophyl.  But  when  the 
flowers  develope,  this  part  of  the  plant  becomes  an  apparatus 
of  combustion.  The  starch  granules  which  in  the  leaves  were 
changed  into  chlorophyl,  in  the  petals  are  changed  into 
chromule,  and  become  wholly  oxidized  and  converted  into 
saccharine  matter.  The  carbon  or  sugar  accumulated  by  the 
nutritive  organs  of  the  plant,  is  consumed  by  its  reproductive 
organs.  Hence  we  see  these  matters  disappear  at  the  epoch 
when  the  flowers  expand,  and  it  is  therefore  necessary  to  reap 
those  vegetables,  which  we  cultivate  for  the  sugar  which  they 
contain,  before  that  period.  This  disappearance  of  the  saccha- 
rine store  is  the  result  of  its  slow  combustion,  or  the  conver- 
sion of  the  carbon  of  the  sugar  into  carbonic  acid.  Oxygen  is 
therefore  necessarily  consumed  and  heat  evolved  by  the  flowers, 
whilst  at  the  same  time  carbonic  acid  rises  from  them  into  the 
atmosphere.  Whilst,  therefore,  the  green  leaves  of  plants 
purify  the  air,  their  beautiful  flowers  contaminate  it,  although 
to  a  degree  of  course  which  is  relatively  insignificant. 

The  development  of  heat  by  flowers  was  first  observed  by 
Lamarck  in  the  Arum  rnaculatum  of  Europe.  It  was  afterwards 
detected  by  Saussure,  in  the  Bignonia,  Gourd,  and  Tuberose- 
In  these  cases  the  heat  was  measured  by  a  common  ther- 
mometer. But  since  the  invention  of  thermo-electric  instru- 


138  COMPOUND  ORGANS  OP  PLANTS. 

ments,  heat  can  be  detected  in  any  ordinary  cluster  of  flowers. 
The  best  plants  for  experiment  are  the  Aracese,  where  the  heat 
is  confined  and  reveberated  by  the  hood-like  inflorescence.  In 
some  of  these  plants  the  temperature  rises  at  times  to  20°  and 
50°  Fahrenheit,  above  that  of  the  surrounding  air.  The  tem- 
perature rises  from  the  first  opening  of  the  flowers,  and  reaches 
its  maximum  when  they  shed  their  pollen,  at  which  time  the 
heat  developed  is  so  great  as  to  be  perceived  by  the  hand ;  it 
afterwards  gradually  declines  until  the  flowers  fade. 


CHAPTER    XI. 

THE   ESSENTIAL  REPRODUCTIVE   ORGANS. 
THE   ANDR03CIUM    OB    STAMINAL  ORGANS. 

THE  stamens  are  situated  immediately  within  the  corolla,  and 
form  the  third  verticil  of  the  flower.  They  constitute,  collec- 
tively, the  andrcecium  (avyp  a  male,  and  oixlov  habitation),  or 
the  male  sexual  organs  of  the  plant. 

There  is  a  power  given  to  all  plants  of  developing  new  plants 
out  of  any  of  their  cells,  when  these  cells  are  placed  in  suitable 
circumstances.  In  the  cells  of  plants  in  general  the  expression 
of  this  law  seldom  occurs,  since  it  is  only  in  rare  cases  that  the 
necessary  conjunction  of  all  the  conditions  is  brought  about. 
Nevertheless,  there  are  cases  in  which  the  ordinary  leaves  of 
the  stem  may  be  made  to  develope  new  plants,  as,  for  instance, 
the  leaves  of  Bryophyllum  calycinum  which,  when  placed  on 
moist  earth,  develope  young  plants  from  the  indentations  of  their 


THE   ANDRCECIUM.  139 

margin.  So,  also,  if  a  notch  is  made  in  one  of  the  thick  veins 
of  the  leaves  of  the  splendid  Gresneria,  and  if  the  leaf  is  placed 
on  the  ground,  in  about  a  week  a  new  plant  will  be  pro- 
duced on  its  surface.  The  same  phenomena  occur  in  the  leaves 
of  the  beautiful  and  scarlet-flowered  Echeverias,  and  in  many 
other  succulent  plants.  Now  these  plants  could  only  originate 
in  the  extraordinary  development  of  certain  cells  in  the  leaf. 

In  general,  however,  those  plants  which  have  true  leaves  and 
flowers,  have  these  cells  always  produced  in  their  terminal 
leaves,  which  at  this  time  take  a  peculiar  form,  as,  for  instance, 
in  the  stamens.  These  reproductive  cells,  which  are  termed 
pollen,  are  always  developed  in  the  interior  of  these  metamor- 
phosed leaves  or  stamens. 

A  stamen,  when  complete,  consists  of  three  parts ;  the  fila- 
ment, or  thread-like  portion,  /•  the  anther,  a,  which  is 
situated  on  the  top  of  the  filament,  and  which  usually  consists 
of  two  cells  placed  side  by  side,  and  attached  to  a  prolongation 
of  the  filament  called  the  connectivum  or  connective ;  and 
the  pollen,  or  granular  matter,  p,  contained  in  the  cells  of  the 
anther,  by  means  of  which  the  ovules  are  impregnated, 
(Fig.  42.)  The  stamens  are  very  conspicuous  in  the  garden 

Fig.  42. 


lily,  an  examination  of  which  flower  will,  in  connection  with 
our  engraving,  convey  a  very  accurate  conception  of  these 
important  organs* 


140  COMPOUND  ORGANS  OF  PLANTS. 

A  fully  developed  leaf  is  composed  of  two  parts,  a  little  stalk 
or  support  called  a  petiole,  and  a  flat  expanded  portion  called 
the  blade  or  limb,  which  is  composed  of  woody  fibre  and  paren- 
chyma. The  veins  of  the  leaf  constitute  its  woody  fibre  and 
form  its  framework  or  skeleton,  whilst  the  parenchyma  is  the 
green  cellular  matter  which  fills  up  the  interstices  or  intervals 
between  the  veins.  Now  the  petiole  of  the  leaf  is  represented 
in  the  stamen  by  the  filament ;  the  midrib  by  the  connectivum ; 
whilst  the  anther  corresponds  to  the  lamina  or  blade,  each 
portion  of  the  lamina,  on  either  side  of  the  connectivum  or 
midrib,  forming  an  anther  lobe.  The  pollen  contained  in  the 
anther-cells  results  from  a  peculiar  transformation  of  the 
parenchyma  or  green  cellular  matter  of  the  leaf. 

When  the  stamen  is  destitute  of  a  filament,  the  anther  is 
said  to  be  sessile,  the  filament  being  no  more  essential  to  the 
stamen  than  the  petiole  to  the  leaf.  When  the  anther  is 
imperfect,  abortive,  or  wanting,  the  stamen  is  considered  to  be 
sterile,  abortive,  or  rudimentary,  its  real  nature  being  known 
by  its  situation. 

In  the  stamens,  the  leaf  undergoes  such  extensive  structural 
changes  that  its  parts  can  scarcely  be  recognized.  That  the 
stamens  are  only  leaves  which  have  undergone  a  greater 
metamorphosis  or  change  of  form,  nature  herself  teaches.  All 
will  allow  the  analogy  of  the  petal  to  the  leaf.  Now,  the  con- 
version of  stamens  into  petals  is  a  common  occurrence  in  plants 
which  have  numerous  whorls  of  stamens,  especially  when  such 
plants  'are  brought  under  cultivation,  as,  for  example,  in  the 
rose  and  peony ;  but  in  no  plant  is  it  seen  more  clearly  than 
in  the  flower  of  the  Nymphsea  alba,  or  white  water-lily.  In 
this  flower,  perfect  stamens  are  formed  in  the  centre,  the 
filaments  of  which  gradually  enlarge  towards  the  circumfe- 


THE   ANDRCECIFM. 


141 


rence,  until  at  length  the  outer  whorls  of  stamens  exactly 
resemble  petals,  except  in  having  their  tops  developed  into 
yellow  anthers,  as  seen  at  a  and  b  in  (Fig.  43;)  and  finally  the 

Fig.  43. 


anther  disappears  altogether  from  the  summit  of  the  petal,  as 
at  c,  and  the  metamorphosis  is  completed. 

In  this  manner,  what  are  called  double  flowers  are  produced. 
The  numerous  whorls  of  colored  petals  in  the  rose  and  peony 
result  from  a  metamorphosis  of  a  part,  or  sometimes  of  the 
whole  of  their  stamens  into  petals.  This  metamorphosis  is  the 
effect  of  cultivation,  the  normal  number  of  petals  in  the  rose 
being  five,  as  is  seen  in  the  wild  roses.  A  double  flower, 
therefore,  although  an  object  of  admiration  to  the  gardener,  is 
nevertheless  justly  regarded,  scientifically,  as  a  monstrosity. 

If  all  the  stamens  are  converted  into  petals,  the  flower  is 
13 


142          COMPOUND  ORGANS  OP  PLANTS. 

necessarily  sterile ;  but  if  some  of  the  stamens  are  perfect, 
even  in  a  double  flower,  there  may  be  fruit. 

The  number  of  stamens  which  compose  the  andrcecium 
varies  very  considerably.  There"  may  be  only  one,  as  in  Calli- 
triche  verna,  Water  star  grass,  or  many  hundreds  as  in  the 
poppy.  The  flower,  according  to  the  number  of  its  stamens 
from  one  to  ten,  is  said  to  be  monandrous  (^6vo?  one,  avrjp  male,) 
diandrous  (5tj  two,)  triandrous  (tpsis  three,)  tetrandrous  (tetpas 
four,)  pentandrous  (itfvte  five,)  hexandrous  (2§  six,)  heptan- 
drous  (Jrt-ra  seven,)  octandrous  (6xtu  eight,)  enneandrous  (fWta 
nine,)  decandrous  (filxa  ten.)  Above  ten  there  is  no  regularity 
in  the  number  of  the  stamens.  All  flowers  having  from  twelve 
to  twenty  stamens,  are  designated  as  dodecandrous  (fiwS sxa, 
twelve;)  and  if  their  number  exceeds  twenty,  Polyandrous 
(rtoXuj  many.) 

Proportion  of  the  stamens. — The  relative  length  of  the 
stamens  is  not  always  the  same,  the  filaments  being  sometimes 
more  or  less  developed  in  the  same  flower.  In  some  cases 
there  exists  a  definite  relation  as  regards  number  between  the 
long  and  the  short  stamens.  When  a  flower  encloses  four 
stamens  of  which  two  are  constantly  the  longest,  it  is  called  a 
didynamous  flower,  (Stj  twice,  and  Swa/us  power;)  Fig.  44 ; 
and  when  there  are  six  stamens  in  the  same  flower  and  four  of 
them  longer  than  the  other  two,  the  flower  is  said  to  be  tetra- 
dynamous,  (tsipas  four,  and  &vva/u$  power ;)  Fig.  45.  The 
natural  orders  Labiatae  and  Scrophulariacese  furnish  us  with 
samples  of  the  first,  and  Cruciferae  of  the  last  disposition  of 
the  stamens.  In  the  wood  sorrel,  (Oxalis,)  there  are  ten 
stamens,  monadelphous  at  their  base,  five  long  and  five  short, 
which  alternate  with  each  other. 

Connexion  of  the  stamens. — The  stamens,  in  common  with 


THE   ANDR(ECIUM.  143 

Fig.  44.  Fig.  45. 


the  other  leaves  of  the  plant,  are  found  in  a  state  of  cohesion 
in  many  flowers.  When  they  cohere  by  their  filaments  to  a 
greater  or  less  extent,  forming  a  tube  around  the  pistil,  as  in 
the  oxalis  and  mallow,  (Malva,)  Fig.  46,  they  are  called  mona- 

Fig.  46. 


Fig.  46.     Vertical  section  of  the  flower  of  Mallow  (Malva.)     The  stamens  are 
monadelphous,  being  united  by  their  filaments  into  a  cluster  round  the  pistil. 


COMPOUND  ORGANS  OP  PLANTS. 


delphous  stamens  (^uoVoj  one,  and  d5f^6j  brotherhood;)  diadel- 
phous,  when  the  filaments  are  united  into  two  bundles,  as  in 
the  pea  and  fumitory.  In  the  latter  instance,  the  same  number 
of  filaments  cohere  together  in  the  two  bundles,  each  of  them 
being  composed  of  three  stamens,  but  in  nearly  all  papiliona- 
ceous flowers,  out  of  ten  stamens  nine  are  united  by  their 
filaments  while  one  is  free.  When  the  filaments  cohere  into 
three  bundles,  the  stamens  are  triadelphous,  as  in  the  St. 
John's-wort,  (Hypericum,)  Fig.  47;  and  when  they  grow 
together  into  many  bundles,  polyadelphous,  as  in  Ricinus  com- 
munis,  the  Castor  oil  plant,  Fig.  48. 

Fig.  47.  Fig.  48. 


Fig.  47.  Vertical  section  of  St.  John's-wort  (Hypericum.)  This  flower  has  tria- 
delphous stamens,  and  a  tricarpellary  pistil.  Only  two  of  the  hundles  of  stamens 
are  visible,  the  third  having  been  removed  along  with  a  part  of  the  pistil. 

Sometimes  the  stamens  adhere  to  each  other  by  their  anthers, 
the  filaments  being  free,  they  are  then  said  to  be  syngenesious  or 
synantherous  (avv  together,  and  ysvs ois  origin,  or  cw/^pa anthers). 
This  kind  of  union  occurs  in  Composite  flowers,  of  which  the 
cichory-is  a  sample,  (Fig.  49.)  Occasionally,  however,  the 
union  of  the  stamens  takes  place  through  their  entire  length, 
their  filaments  as  well  as  their  anthers  cohering,  as  in  Lobelia, 
(Fig.  50.)  At  length  the  androecium,  instead  of  forming  a  dis- 
tinct verticil  about  the  pistil  occupying  the  centre  of  the  flower, 


THE   ANDRCECIUM. 


145 


becomes  united  with  it  so  as  to  constitute  but  one  body. 
In  this  last  case  the  stamens  are  gynandrous  (  ywvj  a  female, 
and  dv»7p  a  male),  and  the  central  body  or  column  is  called  the 
gynostemium  (ywri  pistil,  and  a^/uw  a  stamen),  as  in  Aristolo- 
chia,  (Fig.  50.) 

Fig.  50. 


Fig.  49. 


Fig.  51. 


Fig.  51.     Gynandrous  stamens  of  Aristolochia  rotunda,    a.  The  ovary.    6.  The 
gymnostemium.    c.  The  six  stamens    d.  The  six  lobes  of  the  stigma. 

Let  us  now  examine  briefly  the  parts  of  which  the  stamen  is 
composed,  having  viewed  them  collectively.  We  have  seen 
that  a  fully  developed  stamen  is  composed  of  a  petiole  termed 
a  filament :  a  limb  or  blade  named  an  anther,  the  pulverulent 
parenchyma  contained  in  the  anther  being  called  pollen. 

The  filament  or  petiole  of  the  stamen  supports  the  anther  or 
metamorphosed  lamina  of  the  leaf,  and  commonly  justifies  its 
name  from  its  form,  that  is  to  say,  it  is  generally  filiform  and 
slender.  Sometimes,  however,  it  is  dilated  and  petaloid,  as  in 
Ornithogalum  umbellatum,  the  Star  of  Bethlehem,  a  white 
flower  with  a  bulbous  root,  quite  common  in  meadows  and 

pastures  about  the  middle  of  Spring. 

13* 


146  COMPOUND   ORGANS   OP    PLANTS. 

Filaments  are  usually  of  a  white  color,  but  occasionally 
they  take  the  same  hues  as  the  corolla.  In  Tradescantia  Vir- 
ginica,  the  spiderwort,  the  filaments  are  blue ;  in  the  different 
varieties  of  the  Fuchsia  or  lady's  ear-drop,  they  are  red,  and  in 
Ranunculus  acris,  yellow. 

The  anther  is  generally  situated  at  the  summit  of  the  fila- 
ment, to  which  it  is  attached  in  a  variety  of  ways.  Sometimes 
it  adheres  to  the  filament  by  its  entire  length,  when  it  is  said 
to  be  adnate,  as  in  Magnolia  glauca ;  or  its  base  rests  directly 
on  the  apex  of  the  filament,  when  it  is  innate,  as  in  Sangui- 
sorba  Canadensis,  or  burnet ;  or  it  may  be  attached  by  a  point 
to  the  apex  of  the  filament  on  which  it  lightly  swings,  when  it 
is  versatile,  as  in  the  grasses. 

The  anther  is  the  most  essential  part  of  the  stamen.  It  con- 
tains the  pollen  or  fecundating  matter,  before  the  act  of  fecunda- 
tion. It  is  most  generally  formed  of  two  little  pouches  or  cells 
supported  against  each  other  by  one  of  their  sides,  or  united 
together  by  an  intermediate  body,  to  which  the  name  connec- 
tivum  or  connective  has  been  given.  In  this  case  the  anther 
is  bilocular,  (bis,  twice,  loculus,  a  pouch.)  More  rarely  the 
anthers  are  unilocular,  as  in  the  mallow,  or  quadrilocular,  as  in 
Butomus  umbellatus,  the  flowering  rush ;  a  plant  occasionally 
met  with  in  England  in  brooks  and  rivulets. 

The  pollen  or  fecundating  matter,  when  artificially  removed 
from  the  anther  cells,  looks  to  the  naked  eye  like  powdery 
matter  devoid  of  all  organization,  and  is  usually  of  a  yellow 
color;  but  it  is  also  purple,  blue,  scarlet,  black,  and  various 
other  shades.  Placed  beneath  ^the  microscope,  this  powder 
resolves  itself  into  a  collection  of  spherical  or  oval  grains,  the 
surfaces  of  which  are  generally  smooth,  but  sometimes  fur- 
nished with  strong  points  or  bristles,  as  in  the  hollyhock, 


THE   ANDRCECIUM.  147 

(Althea.)  In  most  plants  these  grains  are  free  amongst  them- 
selves ;  but  in  the  Fuchsia  and  ^Enothera  biennis,  or  evening 
primrose,  they  are  held  together  by  slender  threads,  and  in 
other  genera  they  adhere  together  in  masses  called  pollinia. 

Pulverulent  pollen.  This  is  its  most  general  aspect  and 
disposition.  Pollen  cells  are  ordinarily  composed  of  two 
membranes,  which  are  distinguished  as  external  and  internal. 
The  interior  of  the  cells  is  filled  with  a  mucilaginous  fluid 
matter,  containing  granules,  named  fovilla.  The  exterior 
membrane  of  the  pollen  cell,  denominated  the  extine,  (cxto, 
to  stand  out,)  is  thick,  firm,  and  is  readily  ruptured  by  dis- 
tension. It  is  this  membrane  which  is  covered  with  papillae 
or  granulations,  the  surface  of  the  pollen  being  rarely  smooth. 
It  is  applied  immediately  on  the  internal  membrane,  or  intine, 
(intus,  within.)  This  membrane  is  thin,  transparent,  very 
extensible,  and  without  any  appreciable  organization. 

The  mucilaginous  fluid  and  granular  matter  in  the  interior 
of  the  pollen  cells  has  been  the  object  of  a  great  deal  of  discus- 
sion amongst  physiologists.  The  fovilla  exhibit  very  marked 
movements  in  the  fluid  where  they  swim.  These  movements, 
it  was  at  first  thought  were  spontaneous,  and  the  pollenic 
granules  were  supposed  to  be  assimilated  by  them  to  the 
zoosperms  of  animals.  But  the  analogy  has  been  completely 
destroyed  by  an  examination  of  the  chemical  nature  of  these 
bodies,  which  are  nothing  but  grains  of  starch,  turning  blue 
with  iodine,  and  showing  all  the  characters  of  the  fecula  taken 
from  the  other  parts  of  the  plant.  This  observation  is  due  to 
M.  Fritsch  of  Berlin,  who  published  in  1832  and  1833  two 
interesting  dissertations  on  pollen. 

Solid  pollen  is  that  in  which  the  grains  instead  of  being 
distinct  are  united  together  in  masses,  which  in  general  take 


148  COMPOUND   ORGANS   OP    PLANTS. 

the  form  of  the  cells  of  the  anther  which  serves  as  a  kind  of 
mould.  The  name  pollinia  has  been  given  to  these  agglomera- 
tions. It  is  only  in  the  family  of  Orchidaceae  amongst 
Monocotyledons,  and  that  of  Asclepiadacae  in  Dicotyledons 
that  we  observe  solid  pollen. 

In  orchideous  plants  each  of  the  pollen  masses  is  supported 
on  a  stalk  called  a  caudicle  (cauda  a  tail),  which  carries  at  its 
extremity  a  glandular  body  called  a  retinacula  (retinaculum  a 
band  or  rein),  by  means  of  which  it  is  attached  to  the  stigma. 
These  masses  when  bruised  divide  into  grains  which  are 
agglutinated  together  in  fours. 

Fig.  52. 


Fig.  52.  a.  Represents  one  of  these  pollen  masses,  with  its  caudicle.  &.  The  reti- 
nacula. c.  Some  of  the  grains  separated  from  a  similar  mass  to  show  the  nature  of 
their  agglomeration. 


CHAPTEE    XII. 

THE   GYMNCECIUM   OB  PISTILLINE   ORGANS. 

• 

THE  pistil  occupies  the  centre  of  the  flower  and  terminates 
the  axis  of  growth.  The  pistils  constitute  collectively  the 
Gymno3cium  (yw*i  pistil,  and  oixlov  habitation,)  or  female 
sexual  organs  of  the  plant. 


THE   GYMNCECIUM.  149 

When  fully  developed  the  pistil,  like  the  stamen,  consists  of 
three  parts,  the  stigma,  the  style,  and  the  ovary  (Fig.  53.) 

Fig.  53. 


The  ovary  a,  is  the  lower  part  of  the  pistil,  containing  within 
its  cavity  the  ovules  or  rudimentary  seeds  d.  and  forms  after 
the  impregnation  of  the  ovules  the  future  seed  vessel.  The 
apex  of  the  ovary  usually  tapers  into  a  slender  column  called 
the  style  6,  the  summit  of  which  is  .commonly  somewhat 
enlarged,  denuded  of  cuticle,  and  secretes  a  viscid  matter  to 
which  the  pollen  grains  adhere.  This  denuded  and  glandular 
summit  of  the  style  is  termed  the  stigma,  c. 

The  ovary  and  stigma  are  never  absent,  the  style  sometimes 
is ;  in  which  case  the  top  of  the  ovary  itself  is  called  the  stigma, 
as  in  the  poppy,  where  it  appears  like  the  spokes  of  a  wheel. 

Like  the  other  organs  of  the  flower,  the  pistil  is  composed  of 
one  or  more  modified  leaves,  which  in  this  instance  are  called 
carpels,  from  their  connexion  with  the  fruit,  (xaprtoj,  fruit.) 
These  leaves  are  folded  inwardly,  and  their  margins  united,  so 
that  their  lower  surface  forms  the  outside,  and  their  upper 
surface  the  inside  of  the  carpel,  the  ovules  being  developed 
along  the  margin  of  the  leaves.  That  this  is  the  true  nature 
of  the  pistil,  the  monstrous  variety  of  the  garden  cherry  con- 
clusively proves.  In  this  flower,  the  place  of  the  pistil  is 


150 


COMPOUND  ORGANS  OF  PLANTS. 


occupied  by  a  green  leaf,  somewhat  folded  together,  and  similar 
to  the  leaves  of  the  branches,  except  in  its  lesser  size.  If  we 
compare  this  leaf,  with  the  perfect  pistil  of  the  cherry,  we 
shall  see  that  the  folded  lamina  answers  to  the  ovary,  the 
midrib  projecting  beyond  the  ovary  to  the  style,  and  its 
slightly  dilated  apex  to  the  stigma.  The  analogy  of  carpels  to 
leaves  may  also  be  deduced  from  their  similarity  in  texture 
and  venation,  and  from  the  situation  of  the  ovules,  which 
exactly  corresponds  to  that  of  the  germs  or  buds  found  on 
the  margin  of  some  leaves,  as  on  those  of  Bryophyllum  caly- 


cmum. 


Fig.  54. 


The  modified  leaves  or  carpels  forming  the  gymnaecium, 
cohere  together  to  a  greater  or  less  extent,  like  the  parts  of 
the  flower ;  and  all  degrees  of  union  amongst  them  may  be  ob- 
served from  the  mere  cohesion  of  the  contiguous  bases  of  their 
ovaries,  (Fig.  54,  a)  to  their  perfect  consolidation  whilst  their 
styles  are  distinct,  b.  In  other  species,  both  the  ovaries  and 
styles  of  the  carpels  are  consolidated,  and  the  whole  gymnoe- 
cium  forms  an  unique  body,  which  may  be  mistaken  for  a  single 
pistil,  c.  But  single  pistils  are  by  no  means  so  common  as  is 
usually  supposed.  If  we  make  a  transverse  section  of  the  ovary 


THE   GYMN(ECITJM.  151 

of  this  apparently  single  pistil,  we  shall  find  a  number  of  cells, 
which  are  in  general  equal  to  the  number  of  consolidated  car- 
pels or  pistils.  If  the  ovary  of  the  lily,  for  example,  be  cut 
in  this  manner,  what  appears  at  first  view  to  be  a  single  pistil 
will  be  found  in  reality  to  consist  of  three  united  ones. 

When  the  carpel  and  pistils  of  the  gymnoacium  are  all  distinct 
the  pistil  is  termed  apocarpous,  (airtb  separate,  and  xaprto$  fruit,) 
when  they  are  united  into  one  mass  it  is  said  to  be  syncarpous, 
(avv  together  or  united.) 

Let  us  now  carefully  examine  the  different  parts  of  the  pistils. 

The  ovary  is  the  inferior  part  of  the  carpel  or  pistil,  and 
contains  the  ovules  within  its  cavity.  It  is  either  simple  or 
compound.  Simple  when  it  is  unilocular  or  one-celled ;  com- 
pound when  it  is  bi-locular,  tri-locular,  &c. 

The  partitions  which  divide  the  compound  ovary  into  cells 
are  termed  dissepiments  (dissepio  I  separate) ;  and  each  dissepi- 
ment being  formed  of  the  united  and  contiguous  walls  of  two 
carpels,  necessarily  consists  of  two  layers,  one  belonging  to  each 
carpel,  the  ovary  containing  as  many  cells  as  there  are  carpels 
in  the  compound  pistil. 

The  placenta  is  the  line  or  ridge  to  which  the  ovules  are 
attached,  and  corresponds  to  the  ventral  suture  or  line  formed 
by  the  union  of  the  margins  of  the  carpellary  leaves. 

The  simple  pistil  has  of  course  a  one-celled  ovary,  but  not 
unfrequently  the  ovary  of  the  compound  pistil  is  also  unilo- 
cular. For  the  edges  of  the  carpellary  leaves  are  sometimes 
folded  inwardly,  and  form  imperfect  dissepiments  which  pro- 
ject more  or  less  into  the  cavity  of  the  ovary  but  do  not  divide 
it  into  cells.  In  this  case  the  ovary  is  necessarily  unilocular, 
although  it  may  be  connected  with  a  compound  pistil. 

If  we  suppose  a  circle  of  three  carpellary  leaves  with  their 


152  COMPOUND   ORGANS   OP    PLANTS. 

margins  turned  inwards,  yet  not  so  as  to  meet  in  the  centre  of 
the  ovary,  to  cohere  merely  by  their  contiguous  inflexed  por- 

Fig.  55. 


tions,  a  one-celled  tri-carpellary  ovary  would  result,  with  three 
imperfect  dissepiments  projecting  into  its  cavity,  in  Fig.  55,  a. 
If  we  imagine  the  margins  of  three  carpellary  leaves  to  cohere, 
making  only  three  slight  introflexions,  it  is  obvious  that  there 
would  be  no  dissepiments,  and  the  placentas  would  be  truly 
parietal  (paries  a  wall)  the  ovules  being  borne  directly  on  the 
walls  of  the  ovary,  as  at  b.  If,  on  the  contrary,  we  suppose 
the  three  carpellary  leaves  to  be  so  folded  inwardly  as  to  carry 
the  inflexed  portions  of  their  united  lamina,  or  in  other  words, 
their  dissepiments  to  the  centre,  and  the  dissepiments  there 
to  unite  and  form  a  common  axis,  about  which  the  ovules 
develope;  and  if  we  then  imagine  the  walls  of  the  dissepiments 
to  be  ruptured  by  the  rapid  growth  of  the  ovary,  it  is  obvious 
that  we  shall  have  what  is  called  a  free  central  placenta,  as 
shown  at  c,  Fig.  55,  and  also  in  Fig.  56.  In  all  these  cases  the 
compound  pistil  has  an  unilocular  ovary. 

All  gradations  may  be  observed  in  nature  between  strictly 
parietal  placenta  and  those  which  are  carried  forward  so  as  to 
meet  in  the  centre  of  the  ovary  and  separate  its  cavity  into 
distinct  cells. 

la  the  Dog's-tooth  violet  (Erythronium)  and  Campanula  the 
walls  of  the  dissepiments  are  not  ruptured.  Fig.  55  is  a  tra- 


THE   GYMN(ECIUM.  153 

Fig.  55.  Fig.  56. 


Fig.  56.  Vertical  section  of  the  tricarpellary  ovary  of  Spergularia  rubra,  a  plant 
belonging  to  the  chickweed  family,  showing  the  attachment  of  the  ovules  to  a  free 
central  placenta. 

verse  section  of  the  trilocular  or  three-celled  ovary  of  the  Ery- 
thronium.  The  ovules  are  attached  to  a  central  placenta.  In 
this  instance  the  compound  character  of  the  ovary  is  sufficiently 
evident. 

In  the  chickweed  family,  Fig.  56,  the  dissepiments  at  first 
project  across  the  cavity  of  the  ovary  and  meet  in  its  centre, 
but  are  finally  torn  asunder  by  the  expansion  of  the  ovary,  so 
that  the  several  loculi  communicate,  the  ovules  remaining 
attached  to  the  placentas  in  the  middle.  The  vestiges  of  the 
dissepiments  remain  attached  to  the  walls  of  the  ovary,  proving 
that  this  is  the  mode  in  which  free  central  placentations  are 
produced.  In  the  blood-root  and  violet,  the  placenta  are  strictly 
parietal. 

In  most  cases  the  compound  pistil,  provided  with  a  one-celled 
ovary,  is  easily  recognized.  Thus  every  time  that  an  unilocular 
ovary  is  surmounted  by  several  free  styles  and  stigmas,  or  by 
the  same  united  amongst  themselves  and  only  distinguishable 
at  their  summit  by  some  slight  incision,  the  pistil  will  be  com- 
pound. It  is  only  necessary  to  remember  that  a  pistil  is  never 
without  an  ovary  and  stigma,  and  in  most  cases  possesses  a 

H 


154  COMPOUND  ORGANS  OP  PLANTS. 

style;  the  plurality  of  styles  and  stigmas  therefore  necessa- 
rily proves  a  plurality  of  pistils. 

In  general  the  carpels  contract  no  adhesion  with  the  floral 
envelopes.  They  are  simply  attached  to  the  receptacle,  £0  that 
when  they  grow  and  elevate  themselves  they  remain  perfectly 
intact.  We  say  in  this  case  that  the  ovary  is  free  and  superior, 
being  situated  above  the  floral  envelopes,  and  that  the  stamens 
are  perigynous  (jtspi  around,  and  yvv^  pistil.)  But  sometimes 
the  calyx  grows  to  the  surface  of  the  ovary  carrying  with  it 
the  petals  and  stamens,  so  that  all  these  organs  seem  to  rise  as 
it  were  out  of  the  summit  of  the  ovary,  as  in  the  honeysuckle 
and  dog-wood.  The  ovary  in  this  instance  is  inferior,  as  it  is 
situated  below  the  floral  envelopes,  and  the  stamens  epigy- 
nous  (trft  upon,  yw/i  pistil.)  This  distinction  between  the  infe- 
rior and  superior  ovary  is  very  important,  as  it  serves  to 
distinguish  certain  natural  families. 

The  Style. — The  general  character  of  the  style  in  simple 
ovaries  has  been  already  described.  In  compound  ovaries  there 
are  as  many  styles  as  there  are  carpels;  and  they  either  remain 
distinct,  as  in  the  pink,  or  become  partially  united,  as  in  the 
geranium,  or  completely  consolidated  to  their  summit,  as  in  the 
lily. 

When  we  examine  a  transverse  section  of  the  style  with  a 
sufficient  magnifying  power,  we  always  find  it  hollow.  The 
interior  of  the  style  is  in  fact  a  canal,  extending  from  the 
stigma  to  the  cavity  of  the  ovary.  This  canal  is  sometimes 
open ;-  but  generally  it  is  filled  with  a  humid  and  lax  paren- 
chyma, which  differs  considerably  from  the  other  parenchyma 
of  the  style,  and  which  is  distinguished  as  the  conducting 
tissue.  This  tissue  spread  out  on  the  summit  of  the  style 

rms  that  spongy  surface  called, — 


THE   GYMNCECIUM,  155 

The  Stigma. — This  is  a  glandular  body,  placed  on  the  sum- 
mit of  the  style,  when  there  is  one,  or  immediately  on  the 
ovary  when  there  is  no  style.  It  is  denuded  of  cuticle,  and 
secretes  a  viscid  -fluid  which  detains  the  pollen  grains,  and 
causes  them  to  emit  tubes.  This  secretion  becomes  more 
abundant  as  the  period  of  fecundation  approaches. 

The  stigma  is  simple  when  it  is  connected  with  a  single 
pistil;  but  in  the  compound  pistil  there  are  necessarily  as  many 
stigmas  as  there  are  carpels  united  together.  When  the 
ovaries  and  styles  of  all  the  carpels  of  a  compound  pistil  are 
in  a  state  of  complete  cohesion  and  consolidation,  the  stigma 
always  presents  a  number  of  lobes  or  divisions  more  or  less 
deep,  which  clearly  indicate  the  number  of  pistils  which  have 
cohered  together. 

The  lobes  of  the  compound  stigma  are  excessively  variable ; 
they  may  be  flat  and  pointed,  and  hemispherical  and  blunt, 
smooth,  or  covered  with  salient  papillae,  or  with  hairs  simple 
and  glandular,  or  with  branched  and  plumose  hairs,  as  in  the 


The  Ovule  is  the  body  which  is  contained  in  the  cavity  of 
the  ovary  and  attached  to  the  placenta,  and  which,  after  im- 
pregnation, is  transformed  into  the  seed.  It  experiences  in 
this  transformation  remarkable  changes  in  its  structure,  form 
and  position. 

In  order  accurately  to  trace  the  development  of  an  ovule, 
we  must  commence  our  observations  as  soon  as  the  plant 
begins  to  form  flower-buds.  We  shall  then  see  in  the  interior 
of  the  ovary,  forming  on  the  placenta,  a  minute  excrescence  or 
tubercle,  formed  solely  of  cellular  tissue.  This  gradually  en- 
larges into  a  more  or  less  obtuse  conical  form,  constituting 
what  has  been  called  the  nucleus  of  the  ovule.  As  growth 


156 


COMPOUND   ORGANS    OF   PLANTS. 


progresses,  one  of  the  cells  towards  the  apex  of  the  nucleus 
expands,  forming  a  cavity  in  its  interior,  termed  the  embryo 
sac,  because  it  is  in  this  cavity,  after  impregnation,  that  the 
rudimentary  embryo  first  makes  its  appearance. 

In  the  mistletoe  the  ovule  remains  in  this  simple  and  naked 
condition.  Fig.  57  is  the  ovule  of  the  mistletoe  entire  and  in 
section  with  the  embryo  sac,  c.  In  most  plants,  however,  the 
nucleus  becomes  surrounded  by  one  or  more  coverings  during 
the  progress  of  growth.  These  first  appear  around  the  base  of 
the  nucleus  in  the  form  of  circular  swellings,  which  gradually 
spread  over  its  surface, 

Fig.  57. 


In   some   cases,  as  in  the  ovules  of  the  Walnut,  fig.  59, 
Fig.  58.  Fig.  59. 

f 


the  nucleus  n,  has  only  one  covering  formed  on  its  surface ; 
generally,  however,  whilst  this  envelope  is  increasing,  another 
envelope  jis  formed  outside  of  it,  beginning  at  its  base,  and 


THE   GYMN(ECIUM.  157 

overspreading  its  surface  in  precisely  the  same  way,  as  is  repre- 
sented in  Fig.  59. 

A  fully  developed  ovule,  therefore,  consists  of  a  conically- 
shaped  nucleus  of  cells  containing  a  cavity  or  embryo  sac  in 
its  interior,  with  two  external  coverings.  The  one  s,  next  the 
nucleus  n,  which  is  first  formed,  is  termed  the  secundine,  the 
other  p,  the  primine.  At  the  apex  of  the  nucleus,  both  cover- 
ings leave  an  opening  which  has  been  termed  the  foramen  or 
micropyle,  (juxpoj  little,  rtviq  gate),  through  which  the  nucleus 
slightly  projects  when  it  is  not  completely  covered.  The  open- 
ing or  mouth  of  the  primine,  ex,  is  called  the  exostome,  (fi« 
outside,  and  atopa  mouth;)  that  of  the  secundine  end,  the  en- 
dostome  (evSov  within).  /  is  the  point  where  the  ovule  is 
attached  to  the  placenta. 

The  nucleus  and  its  two  external  investments  have  no  organic 
connexion  with  each  other,  excepting  at  the  base  of  the  ovule, 
where  vessels  pass  from  one  into  the  other  and  unite  the  several 
parts  firmly  together.  This  common  point  of  union  is  termed 
the  chalaza* 

The  ovule  is  attached  to  the  placenta  either  directly,  when 
it  is  said  to  be  sessile,  or  by  means  of  a  prolongation  or  umbi- 
lical cord  termed  the  funiculus,  (funis,  a  cord.)  The  point 
where  this  cord  is  inserted  into  the  ovule  is  termed  the  hilum. 
The  micropyle  or  foramen  is  therefore  situated  at  the  apex  of 
the  ovule,  and  the  chalaza  and  hilum  at  its  base. 

When  all  the  parts  of  the  ovule  develope  uniformly,  they 
maintain  the  same  relative  position  throughout  their  entire 
growth,  as  they  had  at  its  commencement.  Fig.  60.  The 
chalaza  ch,  is  at  the  hilum  or  base  of  the  ovule,  and  the  micro- 
pyle, m,  at  its  apex  or  opposite  extremity,  so  that  a  straight 

14* 


158  COMPOUND  ORGANS  OF  PLANTS. 


line  passes  through  their  axis.  In  this  instance  the  ovule  is 
said  to  be  orthrotropous,  (6p06$,  straight,  and  T-porfoj,  mode.) 

This  is  the  primitive  and  most  simple  form  of  all  ovules, 
although  not  the  most  common.  The  ovules  of  the  Urticaceae 
or  nettle  tribe,  of  the  Cistaceae  or  rock-rose  family,  and  of  the 
Polygonacese  or  buckwheat  family,  are  of  this  character. 

When,  however,  there  is  an  inequality  in  the  development  of 
the  parts  of  the  ovule,  either  one  or  the  other  of  the  following 
modes  of  growth  will  generally  be  the  result. 

Either  the  hilum  and  chalaza  will  remain  together  and  the 
ovule  will  curve  upon  itself,  so  that  the  micropyle  will  be 
brought  near  to  the  hilum,  and  we  shall  have  a  campulitropous 
ovule,  (xanrtvhbs  curved,)  as  in  all  cruciferous  plants,  (Fig.  61 ;) 
or  else  the  chalaza  will  elongate  from  the  hilum  and  become 
transported  to  the  apex  of  the  ovule,  whilst  that  apex  by  an 
inverse  movement  directs  itself  to  the  place  which  the  chalaza 
has  abandoned.  In  this  case,  the  ovule  is  said  to  be  inverted 
or  anatropous,  (watptrtt*  I  subvert.) 

The  curvature  of  the  ovule  in  the  first  instance,  is  to  be 
attributed  to  an  inequality  in  the  development  of  its  sides. 
Thus,  one  of  the  sides  of  the  primine  possesses  more  energy  of 
development  than  the  opposite  side ;  the  former  therefore  elon- 
gates whilst  the  latter  remains  stationary  ;  and  the  resistance 


THE  GYMN<ECIUM. 
Fig.  61.  Fig.  62. 


159 


ch 


Fig.  61.  Campulotropous  ovule  of  Wallflower.  (Cheiranthus.)  /.The  funiculus  by 
which  the  oyule  is  attached  to  the  placenta,  p.  The  primine.  s.  The  secundiue.  n. 
The  nucleus,  ch.  The  chalaza.  The  ovule  is  curved  on  itself,  so  that  the  micro- 
pyle  is  brought  near  to  the  hilum. 

Fig.  62.  Anatropous  ovule  of  Dandelion  (Leontodon).  f.  The  foramen  or  micropyle. 
h.  The  hilum.  ch.  The  chalaza.  n.  The  nucleus,  r.  The  raphe  connecting  the 
chalaza  or  base  of  the  nucleus  with  hilum  h,  and  placenta. 

offered  by  the  inert  side  necessarily  compels  the  extensible  one 
to  turn  round  the  centre  of  resistance,  and  the  ovule  curves 
upon  itself. 

In  the  other  instance,  since  the  hilum  retains  its  place,  the 
vascular  bundle  which  brings  it  into  communication  with  the 
chalaza  is  forced  to  follow  the  ovule  in  its  evolution,  and  forms 
by  its  elongation,  a  cord  more  or  less  prominent  within  the 
thickness  of  the  primine  which  is  called  the  raphe,  (pa^  a 
line.) 

Some  botanists  think  that  the  anatropous  ovule,  is  simply 
an  orthotropous  ovule  inverted  on  an  elongated  funiculus  or 
podosperm,  (nov;  a  foot,  and  <jrt«p/*a  a  seed,)  which  is  attached 
in  the  form  of  a  raphe  to  one  side  of  the  ovule.  But  the 
raphe  r,  Fig.  62,  appears  to  be  an  elongation  of  the  vascular 
bundles  which  connect  the  chalaza  with  the  hilum ;  and  this 
view  is  established  by  the  fact  that  in  anatropous  ovules,  the 


160 


COMPOUND  ORGANS  OF  PLANTS. 


hilum  is  not  seen  at  s,  the  part  where  the  raphe  joins  the 
chalaza,  but  at  h,  the  part  where  it  unites  with  the  placenta. 

These  three  forms  of  ovules  are  by  no  means  clearly  defined 
in  nature,  but  exhibit  varieties,  among  which  we  must  mention 
the  amphitropous  or  heterotropous  ovule,  which  is  produced  by 
a  partial  adhesion  of  the  funiculus  or  raphe  to  the  ovule, 
(Fig.  63.)  The  funiculus  is  seen  at  right  angles  to  the  ovule, 
and  the  hilum  is  placed  midway  between  the  micropyle  and 
chalaza.  The  Leguminosse  or  pea  tribe  have  generally  ovules 
of  this  character. 

Anatropous  ovules  are  the  most  common  in  plants.  The 
orthotropous  form  is  considered  to  be  the  condition  of  all  ovules 
at  the  commencement  of  their  development,  and  the  other 
forms  are  referable  to  changes  produced  during  growth.  The 
anatropous  ovule  of  the  celandine  and  the  campulitropous  •  ovule 
of  the  mallow,  have  been  traced  from  the  orthotropous  condi- 
tion at  the  commencement  of  their  growth,  through  all  the 
intermediate  stages  of  development. 

Fig.  63. 


Fig.  64,  is  a  representation  of  the  development  of  the  anatropous  ovule  of  the 
Celandine,  (Chelidonium  majus.)  1.  and  2,  are  the  first  stages.  The  primine  and  secun- 
dine  investments  are  marked  p  and  «,  and  the  summit  of  the  nucleus  n.  3  is  the  fully 
developed  ovule  after  it  has  executed  its  demi-revolution  on  its  funiculus/.  The 
reason  of  this  singular  change  in  the  position  of  the  ovule  will  appear  in  the  next 
chapter. 

All  these  changes*  in  the  structure,  form  and  position  of  the 
ovules,  are  executed  whilst  the  flower  buds  are  forming.  About 


FERTILIZATION.  161 

the  time  of  the  expansion  of  the  flower,  the  ovules  are  gene- 
rally fully  formed  and  ready  to  receive  the  impregnating  influ- 
ence of  the  pollen.  They  have  become  regularly  shaped 
usually  roundish  bodies  fixed  to  the  placenta  by  one  side. 
They  are  not  yet  seeds,  but  are  destined  to  become  seeds  at  a 
future  period. 


CHAPTER  XIII. 

THE  PROCESS  OF  FERTILIZATION  OR  FECUNDATION. 

Functions  of  the  stamens  and  pistils. — Fecundation  is  that 
function  by  which  the  pollen  is  brought  into  contact  with  the 
pistil,  so  as  to  produce  within  the  ovule  the  formation  of  an 
embryo.  The  results  of  fecundation  are  the  transformation  of 
the  ovules  into  seeds  and  of  the  carpels  into  fruits.  Let  us 
consider, — 

1..  The  preparatory  or  precursory  phenomena  of  fecundation, 
or  the  arrangements  made  for  securing  the  application  of  the 
pollen  to  the  stigma.  Fecundation  in  general  takes  place  at 
the  period  of  anthesis,  (aivdyats,  flower  opening.)  The  anthers, 
up  to  this  time  unruptured,  open  their  cells,  and  spread  the 
pollen  over  the  stigma  and  very  frequently  over  the  other 
parts  of  the  flower,  and  it  is  then  that  fecundation  is  effected. 

There  are  however  a  certain  number  of  plants  among  which 
fecundation  takes  place  before  the  expansion  of  the  floral 
organs.  This  is  the  case  with  many  of  the  Compositse  and 
Aster  tribe  which  have  syngenesious  stamens,  the  stigmas  and 
styles  of  whose  pistils  are  clothed  with  what  botanists  have 


162  COMPOUND  ORGANS  OP  PLANTS. 

agreed  to  call  collecting  hairs.  The  style  of  these  plants  is  at 
first  shorter  than  the  stamens  and  enclosed  by  the  cohering 
anthers;  as,  it  developes  it  pushes  its  way  through  them,  and 
the  hairs  on  its  surface  brush  the  pollen  out  of  the  anther- 
cells,  carrying  it  up  along  with  them.  Hence  when  the 
flowers  are  fully  expanded  we  find  the  anthers  already  open 
and  in  part  empty,  fecundation  having  been  accomplished. 

In  most  cases,  however,  fecundation  does  not  take  place 
whilst  the  perianth  encloses  the  sexual  organs,  but  at  the  time 
of  anthesis.  When  this  period  arrives,  the  opening  of  the  floral 
envelopes  frees  the  stamens  from  all  confinement  and  restraint, 
and  they  take  a  rapid  development.  Their  filaments  elongate, 
and  the  pollen  contained  in  the  anther-cells  up  to  this  period 
succulent,  moist,  and  adherent  to  the  cell  walls,  becomes  dry, 
pulverulent,  and  free  within  their  cavities.  About  this  time 
too,  the  stigma  or  summit  of  the  pistil,  tumefies  and  excretes 
in  great  abundance  a  viscous  fluid  which  lubricates  its  surface 
and  causes  it  to  retain  the  pollen  grains. 

But  before  the  pollen  of  the  stamens  can  be  applied  to  the 
stigma  of  the  pistil,  it  is  necessary  that  it  should  have  some 
outlet  or  means  of  escape  from  the  anther-cells.  In  the 
greatest  number  of  cases,  the  cells  open  longitudinally  through 
the  whole  extent  of  that  furrow  or  groove  which  may  be 
readily  observed  on  their  surface,  as  in  the  gilliflower, 
Sometimes,  however,  the  dehiscence,  (dehisco,  I  gape),  only 
takes  place  at  the  upper  part  of  the  furrow,  by  an  aperture 
resembling  a  pore,  as  in  Pyrola  chlorantha,  (Fig.  66.) 

In  the  common  barberry,  (Berberis,)  the  cells  present  no 
furrow,  but  a  portion  of  their  anterior  surface  opens  in  the 
form  of  valves,  (Fig.  67.)  In  Pyxidanthera  barbulata,  the 


FERTILIZATION. 


163 


anther-cells  open  by  traverse  dehiscence  in  the  form  of  an 
operculum  or  lid,  (Fig.  68.) 


Fig.  66. 


Fig.  67. 


Fig.  68. 


The  mechanical  application  of  the  pollen  to  the  stigma  is 
sometimes  secured  by  certain  relative  adjustments  of  the 
organs.  Thus  when  the  stamens  and  pistils  are  situated  in 
separate  flowers  on  the ~  same  plant,  the  staminate  flowers  are 
generally  situated  above  the  pistillate.  The  Indian  corn 
exemplifies  this  arrangement.  It  is  well  known  that  the 
flowering  panicle  at  the  summit  of  the  stem  does  not  produce 
corn ;  these  are  the  staminiferous  flowers,  from  whose  anthers 
descend  clouds  of  pollen  on  the  thread-like  pistils,  forming  the 
silky  tuft  beneath.  Without  this  pollen,  the  corn  in  the  lower 
spike  w~uld  not  ripen;  hence  the  evident  design  of  nature  in 
placing  the  pistillate  below  the  staminate  spike  of  flowers. 

In  pendulous  and  upright  flowers,  the  filaments  of  the 
stamens  and  the  style  of  the  pistil  are  so  developed  as  to  bring 
the  anthers  and  stigma  into  the  most  favorable  relative  position 
for  communicating  with  each  other.  This  is -beautifully  ex*em- 
plified  in  the  ladies  ear-drop,  (Fuchsia.)  Within  the  pendulous 
corolla  of  this  flower,  we  have  an  adjustment  of  the  sexual 
organs  with  an  evident  reference  to  their  mutual  action  on 
each  other.  The  filaments  of  the  stamens  are  short  and  the 


164  COMPOUND  ORGANS  OF  PLANTS. 

style  of  the  pistil  is  considerably  elongated,  and  its  lubricated 
and  viscid  stigma  is  brought  below  the  anthers  ready  to  receive 
the  falling  pollen.  In  upright  flowers  we  have  a  reverse 
arrangement  of  the  parts;  for  the  style  of  the  pistil  is  in  a 
great  measure  suppressed,  and  the  filaments  of  the  stamens 
are  so  developed  as  to  place  the  anthers  above  the  stigmatic 
surface. 

In  many  plants  fecundation  is  effected  by  certain  special 
movements  of  the  male  or  female  organs  of  the  flower.  The 
flowers  of  the  mountain  laurel  (Kalmia)  are,  in  this  respect, 
especially  deserving  of  examination.  The  corollas  of  the 
Kalmia  are  rotate  or  wheel -shaped,  and  have  ten  stamens. 
The  anthers  of  these  stamens,  before  the  flowers  expand,  are 
contained  in  ten  little  cavities  or  depressions  in  the  side  of 
each  corolla,  where  they  are  secured  by  a  viscid  secretion ; ' 
when  the  corollas  open,  the  filaments  are  bent  back  by  the 
confinement  of  their  anthers,  like  so  many  springs,  in  which 
condition  they  remain  until  the  pollen  in  the  anther-cells 
becomes  ripe,  and  absorbs  the  secretion.  The  anthers  becom- 
ing suddenly  liberated  by  this  means  from  their  confinement, 
fly  up  from  their  cavities  with  such  force  as  to  eject  their 
pollen  on  the  stigma  of  the  pistil.  The  slightest  touch  with 
the  point  of  a  needle,  or  the  feet  of  an  insect  crawling  over 
their  reflexed  filaments,  will  produce  the  same  effects,  if  the 
pollen  is  mature. 

In  the  same  manner,  the  stamens  of  the  common  barberry 
spring  to  the  pistil  if  the  lower  part  of  their  filaments  is 
touched,  and  ^seldom  fail  in  making  the  movement  to  throw  a 
quantity  of  pollen  on  its  stigma.  The  stamens  of  the  Hue,  of 
some  of  the  Saxifrages,  and  of  Parnassia  palustris,  a  rare  and 
beautiful  snow-white  swamp  flower,  do  this  in  succession,  first 


FERTILIZATION,  165 

one  and  then  the  other  approaching  the  pistil  and  discharging 
upon  it  the  polliniferous  contents  of  their  anthers. 

When  grains  of  pollen  are  thrown  on  water,  the  absorption 
of  the  fluid  is  so  rapid,  that  they  burst,  and  a  thick  liquid 
escapes  from  them  which  spreads  itself  over  the  surface  of  the 
water.  This  thick  liquid,  in  fig.  69,  is  seen  escaping  from  one 

Fig.  69. 


of  the  pollen  grains  of  Ipomoea  hederacea,  and  is  the  fecundating 
matter  of  the  grain.  The  action  of  the  pollen  is  therefore 
liable  to  be  frustrated  by  wet  weather.  This  evil  is  guarded 
against  by  the  property  which  the  anther-cells  possess  of  open- 
ing only  in  fine  weather,  as  well  as  by  the  action  of  the  floral 
envelopes,  which  in  some  plants  appear  to  be  exceedingly 
hygrometrical,  enveloping  the  sexual  organs  on  the  slightest 
appearance  of  any  humidity  in  the  atmosphere.  The  flowers 
of  the  red  chickweed  (Anagallis)  are  a  very  remarkable  illus- 
tration of  this  phenomena. 

In  this  view  too  the  economy  of  various  aquatic  plants  is 
exceedingly  interesting,  as  for  instance  the  pond  weeds  (Pota- 
mogeton.)  These  plants  live  wholly  submerged  in  the  water; 
but  at  the  time  of  flowering,  the  peduncles  or  flower  stalks 
elongate  so  as  to  raise  their  flowers  to  the  surface  on  which 
they  may  be  seen  floating.  The  act  of  fertilization  is  thus 
accomplished  in  the  open  air,  and  the  ovaries  are  again  drawn 
beneath  the  water,  where  the  seed  ripens.  The  peduncles  of 

15 


166  COMPOUND   ORGANS   OF    PLANTS. 

the  white  water  lily,  Nelumbium,  and  Brazenia  peltata,  some- 
times attain  the  length  of  from  fifteen  to  twenty  feet  along  the 
shores  of  some  of  the  American  lakes,  so  as  to  bring  their 
flowers  to  the  surface  j  in  fact,  the  length  of  the  peduncle  of 
these  plants  appears  to  be  wholly  regulated  by  the  depth  of 
the  waters  in  which  they  are  found  floating. 

The  essential  phenomena  of  fecundation^  consist  in  those 
changes  which  take  place  in  the  pollen  grains  when  brought 
into  contact  with  the  stigma  of  the  pistil,  together  with  the 
action  of  the  pollenic  tubes  on  the  ovules.  We  have  intimated 
that  pollen  grains  discharged  from  the  anther-cells  on  the 
stigma,  are  retained  there  by  a  viscid  fluid,  which  at  this  time 
most  plentifully  bedews  the  stigmatic  surface.  Very  soon  we 
see  them  swell  out,  as  they  absorb  this  fluid,  those  which 
are  elliptical  or  elongated  becoming  almost  spherical.  At  the 
end  of  a  certain  time,  consisting  of  a  few  hours  for  some  species, 
and  many  days  for  others,  the  thin  and  highly  extensible  iiitine 
or  inner  coat  of  the  pollen  grain  is  seen  protruding  in  the  form 
of  a  tubular  or  vermiform  appendage. 

The  mode  of  dehiscence  of  the  pollen  grains  is  always  deter- 
mined by  their  structure.  Those  which  present  pores,  grooves, 
or  folds  on  their  exterior  surface,  usually  emit  their  tubes  at 
these  points.  The  number  of  tubes  emitted  from  pollen  grains 
is  very  variable ;  sometimes  we  see  only  one,  and  occasionally 
two  or  three,  as  in  the  triangular  pollen  of  the  evening  prim- 
rose (CEnothera)  Fig.  70.  Amici  was  able  to  detect  from 
twenty-six  to  thirty  tubes  which  were  protruded  from  the  same 
cell.  The  number  of  tubes  must  necessarily  bear  some  relation 
to  the  number  of  pores  when  these  exist,  and  we  know  that 
they  are  sometimes  very  numerous.  The  pollen  tubes  are 
filled  with  a  fecundating  fluid  termed  fovillse,  and  it  is  easy  to 


FERTILIZATION.  167 

Fig.  70. 


see  through  their  thin  transparent  walls  the  movements  of  the 
microscopical  corpuscles  which  it  contains. 

As  soon  as  the  pollenic  tubes  have  been  protruded  from  the 
pollen  grain,  they  penetrate  the  loose  cellular  tissue  which 
constitutes  the  mass  of  the  stigma,  known  as  the  conducting 
tissue,  and  insinuating  themselves  amongst  the  interspaces  of 
its  cells  where  they  find  an  abundance  of  moisture,  they  grow 
downwards  through  the  central  part  of  the  style  until  they 
reach  its  base,  a  distance  in  some  cases  of  several  inches. 
Hence  by  making  a  longitudinal  section  of  the  pistil  we  are 
able  to  find  these  tubes  and  to  trace  their  course. 

The  pollen  tubes  may  be  readily  inspected  under  the  micro- 
scope in  many  plants,  and  in  none  more  readily  than  in  the 
Asclepias  or  milkweed.  In  that  family  the  pollen  grains 
cohere  together  in  masses  termed  pollinia,  and  their  united 
tubes  are  protruded,  and  consequently  are  of  such  a  size  as  to 
be  easily  perceived  with  a  very  moderate  magnifying  power. 
Fig.  71. 

The  action  of  the  pollenic  tubes  on  the  ovules.  At  the  time 
that  fecundation  is  operating,  and  the  pollenic  tube  is  being 
elaborated,  the  ovules  are  organized  into  a  suitable  form  for  its 


168  COMPOUND   ORGANS   OF    PLANTS. 


' 

-  • :' , 


Fig.  71.    Pistil  of  Asclepias  a,  with  pollen  masses  p,  adhering  to  the  stigma   s. 
b.  Separate  pollen  masses  united  by  a  gland  like  body. 

reception.  We  have  seen  that  the  nucleus  is  covered  by  two 
membranes,  called  the  primine  and  secundine,  and  that  at  the 
"apex  of  the  nucleus  both  coverings  leave  an  opening  which 
has  been  termed  the  foramen  or  micropyle."  Now  this  open- 
ing, or  the  nucleus  projecting  beyond  it,  is  the  ultimate  desti- 
nation of  the  pollen  tube.  Before  its  arrival,  however,  one  of 
the  cells  towards  the  summit  of  the  nucleus  expands  and  thus 
creates  a  cavity  in  its  interior  which  is  called  the  embryo  sac, 
because  it  is  in  the  interior  of  this  sac  that  the  embryonal 
vesicle  first  makes  its  appearance  in  the  upper  part  of  the 
cavity.  It  is  at  first  a  simple  cell  which  insensibly  elongates, 
and  by  the  formation  of  transverse  septa  forms  itself  into  a  sort 
of  confervoid  tube.  The  terminating  cell  of  this  tube  enlarges 
and  forms  the  embryonal  vesicle.  The  pollen  tube,  having 
arrived  at  the  base  of  the  style,  enters  the  ovary,  and  makes 
its  way  through  the  micropyje  or  orifice  of  the  ovule,  pene- 
trating the  tissue  of  the  nucleus  till  it  reaches  the  embryo  sac. 
Fecundation  appears  to  be  produced  by  the  simple  contact  of 


FERTILIZATION.  169 

the  pollen  tube  with  the  embryo  sac,  and  the  imbibition  by  the 
embryonal  vesicle  of  the  contents  of  the  pollen  grain  through 
the  intervening  membranes,  the  vitally  active  contents  of  the 
two  cells  being  thus  commingled. 

The  development  of  the  embryo.  The  embryonic  vesicle  is 
at  first  simply  a  spherical  cell,  developed  at  the  end  of  the  sus- 
pensory filament,  filled  with  fluid,  and  containing  granular 
matter.  A  little  time  after  fecundation,  a  longitudinal  septum, 
in  the  same  direction  as  the  suspensor,  is  seen  to  form  across 
the  cavity  of  the  cell,  which  thus  becomes  divided  into  two  cells. 
Very  soon  each  of  these  two  cells  is  divided  into  two  others, 
and  all  prove  successively  the  same  segmentation.  The  neces- 
sary result  of  this  is,  a  little  mass  of  cellular  tissue  limited 
exteriorly  by  the  walls  of  the  primitive  cell,  forming  the 
embryonal  vesicle.  It  is  this  mass  of  cellular  tissue  which  by 
degrees  organizes  itself  into  an  embryo. 

In  some  plants  it  remains  in  this  primitive  and  somewhat 
amorphous  state,  being  simply  a  mass  of  cells  without  distinc- 
tion of  organization  or  of  parts.  This  is  its  condition  in  all 
Acotyledonous  or  Cryptogamous  plants/  where  the  embryo 
bears  the  special  name  of  spore.  In  Phanerogamous  plants, 
however,  this  mass  of  cells  assumes  a  more  highly  developed 
state.  The  cells  in  the  upper  part  of  the  mass  which  are 
immediately  connected  with  the  suspensory  filament,  elongate 
into  a  somewhat  conoid  body,  which  in  the  perfect  embryo 
constitutes  the  radicle,  whilst  the  cells  in  the  lower  part  soon 
begin  to  present  traces  of  their  future  cotyledonary  character, 
the  end  farthest  from  the  suspensor  becoming  two-lobed  in 
Dicotyledonous,  and  one-lobed  in  Monocotyledonous  embryos. 
wv,  the  name  of  a  plant  having  leaves  like  seed-lobes.) 
15* 


170  COMPOUND   ORGANS   OF    PLANTS. 

Fig.  72,*  shows  the  different  stages  in  the  development  of 

Fig.  72. 
c  d  e  f          9 


a  b 

Fig.  ,72.  a.  Vertical  section  of  the  pistil  of  Polygonum  after  fertilization,  showing, 
the  pollen  grains  adherent  to  the  stigma  with  their  tubes  passing  down  the  style,  the 
erect  orthotropous  ovule  in  the  interior  of  the  ovary,  and  the  nascent  embryo  sac. 
A  pollen  grain  detached  with  its  tube.  6.  The  pvule  more  highly  magnified,  showing 
the  embryonal  vesicle  formed  in  the  interior  of  the  sac  at  a  later  period,  c.  The  nas- 
cent embryo  and  its  suspensor  removed  from  the  sac,  and  more  magnified,  d, «,/. 
The  embryo  in  succeeding  stages  of  development,  g.  The  embryo  as  it  exists  in  the 
seed. 

the  Dicotyledonous  embryo  of  a  species  of  Polygonum.  Only 
one  ovule  is  contained  in  the  ovary. of  the  pistil,  and  this  is 
orthotropous.  The  plant  has  therefore  been  very  properly 
selected  for  illustration  on  account  of  the  simplicity  of  its 
pistil.  The  process  is  the  same,  although  more  complicated 
when  the  ovules  are  more  numerous. 

The  change  of  the  ovule  into  the  seed.  It  will  be  perceived 
that  as  the  embryo  is  developed,  the  suspensory  filament  by 
which  it  is  attached  to  the  summit  of  the  embryo  sac  is 
gradually  absorbed;  also  that  great  changes  must  necessarily 

*  "Botanical  Text-Book,"  by  Asa  Gray,  M.  D. 


FERTILIZATION.  171 

take  place  in  the  structure  of  the  ovule  as  the  embryo  forms 
in  its  interior. 

In  some  instances,  though  rarely,  all  the  parts  of  the  ovule 
are  visible  in  the  seed ;  but  in  general  these  parts  either  dis- 
appear altogether,  as  the  embryonic  mass  increases  in  bulk,  or 
are  very  materially  altered.  In  many  Dicotyledons,  the  em- 
bryo as  it  develops  absorbs  into  itself  not  only  the  embryo 
sac  but  the  tissue  which  forms  the  nucleus,  so  that  the  seed 
at  its  maturity  contains  nothing  but  an  embryo  of  which  the 
cotyledons  are  thick  and  fleshy,  by  the  amount  of  nutritious 
matter  which  they  have  absorbed,  and  the  integuments  of  the 
ovule,  the  primine  and  secundine,  which  form  its  general 
covering.  This  is  the  case  for  example  in  the  Leguminous 
family.  The  pea  (Pisum)  is  a  good  illustration. 

But  in  other  Dicotyledonous  plants,  and  in  all  Monocotyle- 
dons, the  nutriment  which  the  ovule  contained  in  its  interior 
is  unabsorbed  into  the  embryo,  which  does  not  increase  much 
in  bulk,  and  encroaches  very  slightly  on  the  cells  of  the  nu- 
cleus. These  cells  therefore  become  filled  with  a  deposit  of 
solid  matter  termed  albumen,  in  the  midst  of  which  the  embryo 
is  embedded. 

Seeds  in  which  the  embryo  occupies  the  entire  seed  are 
called  ex-albuminous  (ex  without),  as  the  Compositse,  Cruci- 
ferae,  and  Leguminosae,  whilst  others  having  separate  albumen 
are  albuminous.  The  larger  the  quantity  of  albumen  in  the 
seed,  the  smaller  the  embryo. 

Soon  after  fertilization,  the  pollen  tube  withers  from  above 
downwards,  the  foramen  or  micropyle  of  the  ovule  closes,  and 
when  the  embryo  is  fully  developed  within  it,  the  ovule 
becomes  the  seed  and  the  ovary  the  fruit. 

The  changes  which  manifest  themselves  in  the  flower  and  in 


172          COMPOUND  ORGANS  OP  PLANTS. 

the  other  sexual  organs  of  the  plant  after  fecundation.  A  plant 
in  every  stage  of  its  existence  is  a  beautiful  subject  for  con- 
templation, but  particularly  at  the  close  of  the  period  of  its 
life.  What,  when  its  leaves  are  withering  and  falling  from  its 
stem!  when  its  flowers  are  losing  their  brilliant  hues  and 
inimitable  coloring !  and  when  the  whole  vegetable  economy 
of  the  plant  is  languishing !  Yes,  even  then  it  becomes,  if 
possible,  an  object  of  deeper  admiration.  Why  do  the  leaves 
fall  from  its  stem  ?  Because  food  is  no  longer  required  to  be 
taken  from  the  atmosphere.  Why  do  the  flowers  lose  their 
beauty,  the  petals  detach  themselves  and  fall,  and  even  the 
stamens  experience  tne  same  degradation  ?  It  is  because  these 
parts  of  the  plant  have  fulfilled  their  allotted  functions.  No 
leaf  or  flower  fades  or  falls  in  nature  before  it  has  accomplished 
the  purposes  of  its  creation.  You  see  that  the  pistil  alone 
remains  in  the  centre  of  the  flower.  But  the  style  and  stigma 
are  now  useless  to  the  plant,  and  therefore  they  disappear 
equally  with  the  other  parts.  The  ovary  alone  is  persistent, 
since  it  is  in  its  bosom  that  nature  has  carefully  deposited  the 
embryo  or  seed  which  contains  in  itself  the  rudiments  of  future 
generations. 

A  little  time  after  fecundation,  we  see  the  ovary  increase  in 
size,  the  ovules  which  it  encloses  being  converted  into  seeds 
containing  an  embryo,  and  very  soon  the  ovary  has  acquired 
all  the  characters  proper  to  constitute  it  a  fruit. 


MODIFICATIONS   OF   THE   FLORAL   ORGANS.  173 

CHAPTER    XIV. 

ON   THE   VARIOUS     MODIFICATIONS    OF   THE   FLORAL   ORGANS. 

HITHERTO  we  have  studied  the  flower,  in  the  higher  degrees 
of  its  development,  in  a  complete,  symmetrical,  and  regular  state. 
We  have  to  a  certain  extent  supposed  that  there  was  no  dis- 
turbance of  this  regularity.  Thus  we  have  described  the 
flower  as  composed  of  all  its  verticilsj  the  calyx,  the  corolla, 
the  stamens,  and  pistils — or  in  a  complete  state.  We  have 
supposed  the  parts  of  each  verticil  to  be  alike  in  size  and 
shape,  or  the  flower  to  be  regular,  and  each  verticil  to  contain 
the  same  number  of  pieces  or  a  multiple  of  that  number, 
separate  from  each  other  and  alternating  among  themselves — or 
the  flower  t6  be  symmetrical. 

Fig.  73. 


To  obtain  an  exact  view  of  the  symmetry  of  a  flower  we  must 
observe  it  whilst  in  the  bud,  and  trace  it  out  in  the  form  of  a 
horizontal  section,  as  if  all  the  verticils  had  been  deprived  of 
height  and  sunk  down  to  the  same  plane.  We  are  thus 
enabled  to  see  at  a  glance  the  position  of  the  different  parts 
of  the  flower.  This  theoretical  section  is  called  a  diagram. 
Fig.  73  is  a  diagram  of  a  complete,  symmetrical,  and  regular 


174  COMPOUND  ORGANS  OF  PLANTS. 

flower,  that  of  a  Crassula,  showing  the  alternation  of  the  parts 
of  each  verticil,  and  also  an  equality  in  the  number  of  pieces 
of  which  each  verticil  is  composed.  All  the  verticils  are 
separate  from  each  other,  and  the  parts  of  each  are  equally 
distinct. 

Assuming  this  complete  and  symmetrical  flower  to  be  the 
normal  plan  or  type  on  which  flowers  are  constructed,  when 
we  examine  the  various  plants  around  us  we  find  that  most  of 
them  are  in  an  abnormal  state,  and  we  are  able  only  to  cite  a 
very  small  number  whose  flowers  preserve  this  complete, 
symmetrical,  and  regular  condition.  In  the  immense  majority 
of  cases  the  regularity  is  destroyed,  and  the  symmetry  disguised 
by  a  variety  of  causes.  The  following  are  those  which  act  the 
most  frequently  : — 

One  or  more  additional  verticils  of  the  same  organs  have 
been  developed. — Thus  in  Ranunculus  we  have  five  sepals,  five 
petals,  and  numerous  stamens  and  pistils ;  this  is  occasioned 
by  the  development  of  additional  whorls  of  stamens  and  pistils. 
A  multiplication  of  stamens  also  occurs  in  other  plants,  as  in 
Anemone  and  Hypericum. 

The  composition  of  the  flower  is  somewhat  different  in 
Dicotyledons  and  Monocotyledons.  In  the  first,  it  is  the  num- 
ber five  or  one  of  its  multiples  which  commonly  predominates. 
Thus  the  calyx  is  generally  composed  of  five  sepals,  the  corolla 
of  five  petals,  the  androecium  of  five,  ten,  or  twenty  stamens, 
and  the  gymno3cium  of  five  pistils  or  some  multiple  of  that 
number, -the  parts  of  all  the  extra  verticils  alternating  with 
each  other.  Fig.  74  is  a  diagram  of  the  flower  of  the  Ranun- 
culus with  five  sepals,  five  petals,  and  numerous  stamens  and 
carpels  in  alternating  rows  of  five  each.  This  orderly  distribu- 
tion of  a  certain  number  of  parts  is  called  symmetry,  and  a 


MODIFICATIONS    OF   THE    FLORAL   ORGANS. 

Fig.  74.  Fig.  75, 


175 


flower  in  which  the  parts  are  arranged  in  fives  is  said  to  be 
pentamerous,  (rthts  five,  jts'poj  a  part.) 

In  monocotyledons,  on  the  contrary,  we  observe  more  fre- 
quently the  number  three  or  one  of  its  multiples,  or  the  flower 
is  trimerous,  (ftpsi$  three,  ^'po*  a  part.)  Fig.  75  is  a  good  illus- 
tration. It  is  a  diagram  of  the  flower  of  the  snow-flake, 
(Leucojum),  a  monocotyledonous  plant,  having  three  sepals, 
three  petals,  six  stamens  in  two  alternating  rows,  and  three 
carpels.  This  flower  is  symmetrical,  complete,  regular,  and 
trimerous. 

The  number  of  extra  verticils  which  are  developed  is  some- 
times very  considerable,  as  in  the  Cactus  and  white  water-lily, 
(Nymphaea.)  In  such  circumstances  it  is  easy  to  perceive 
that  the  disposition  by  verticils  is  only  apparent  and  that  the 
floral  leaves  are  arranged  in  a  spiral  about  the  axis  of  growth. 
The  spiral  law  not  only  produces  the  orderly  distribution  of 
the  leaves  about  the  stem,  but  ensures  the  same  symmetry  in 
the  floral  organs,  producing  that  regular,  alternation  of  the 
parts  of  each  verticil,  and  in  these  plants  a  very  perceptible 
spiral  arrangement  of  them. 

The  parts  of  the  floral  organs  may  have  been  increased  by 


176 


COMPOUND   ORGANS   OP    PLANTS. 


deduplication  or  chorization,  (#copt£w,  I  separate,)  that  is,  by 
the  splitting  of  the  organs  during  their  development.  This 
process  accounts  satisfactorily  for  the  appearance  of  certain 
parts  which  do  not  follow  the  law  of  alternation.  This  chorisis 
or  separation  is  either  collateral,  the  separated  parts  being 
placed  side  by  side,  or  transverse,  the  parts  separated  being  left 
one  in  front  of  the  other. 

Of  collateral  chorisis  we  have  a  good  example  in  the  tetra- 
dynamous  stamens  of  the  Cruciferse.  The  stock  and  wall- 
flower belong  to  this  natural  order,  and  are  plants  with  which 
all  are  familiar.  Fig.  76  is  a  diagram  of  a  flower  of  the  corn- 


Fig.  76. 


Fig.  77. 


mon  stock,  (Matthiola  incana,)  showing  a  calyx  with  four 
sepals,  a  corolla  with  four  petals,  but  the  stamens  are  six : 
four  long  and  two  short ;  the  former,  placed  together  in  pairs 
as  shown  in  the  diagram,  are  supposed  to  have  been  originally 


MODIFICATIONS   OF   THE   FLORAL   ORGANS.  177 

one  stamen  which  has  been  split  into  two  by  collateral  chorisis, 
thus  producing  the  want  of  symmetry  in  the  stamineal  circle. 
And  that  this  supposition  has  some  foundation  is  evident  from 
what  we  see  in  Streptanthus  hyacinthoides,  (Fig.  76,)  one  of  the 
wild  flowers  of  Texas.  In  this  plant  the  chorization  has  been 
arrested  before  its  completion,  so  that  in  the  place  of  two 
stamens  we  see  a  forked  filament  bearing  two  anthers. 

The  beautiful  marsh  flower,  called  by  botanists  the  ElodaBa 
Virginica,  (Fig.  77,)  furnishes  us  with  another  sample  of  col- 
lateral chorisis.  The  ground  plan  of  this  flower,  a,  shows  it  to 
be  both  pentamerous  and  trimerous  in  its  organization,  its 
floral  envelopes  consisting  of  five  sepals  and  petals,  whilst  its 
androecium  and  gymnoecium  consist  of  nine  stamens  and  three 
pistils,  the  nine  stamens  being  triadelphous,  and  evidently 
formed  by  collateral  chorization  out  of  three,  as  shown  at  b. 
The  three  glands  which  occupy  an  intermediate  position 
between  the  corolla  and  the  androecium,  as  shown  in  the  dia- 
gram, are  probably  the  rudimentary  traces  of  an  exterior  circle 
of  stamens  which  have  been  rendered  abortive. 

Fig.  78.  Fig.  79.  Fig.  80. 


Transverse  chorization,  or  the  separation  of  a  lamina  from 
organs  already  formed,  is  believed  to  take  place  in  the  case  of 

16 


178  COMPOUND  ORGANS  OF  PLANTS. 

appendages  to  petals.  In  Kanunculus,  transverse  chorization 
or  dilamination  of  the  petals,  produces  a  scale-like  body  at 
their  base,  (Fig.  78,)  and  a  two-lobed  appendage  on  the  inside 
of  the  lamina  of  the  petals  of  Silene,  (Fig.  79  ;)  and  in  Parnas- 
sia  Caroliniana  this  accessory  structure  assumes  at  the  base  of 
the  petal  the  appearance  of  abortive  stamens,  (Fig.  80.)  These 
bodies,  however,  are  situated  opposite  to  the  petals  as  shown  in 
the  diagram,  and  the  stamens  alternate  with  the  lobes  of  the 
corolla  and  are  therefore  in  their  normal  position,  so  that 
these  appendages  are  certainly  not  stamens,  but  are  produced 
by  the  transverse  chorization  or  dilamination  of  the  petals 
opposite  to  which  they  are  placed. 

One  or  more  pieces  of  the  same  verticil  may  have  united 
among  themselves,  or  the  whole  of  the  pieces  of  the  same  verti- 
cils may  have  become  coherent.  These  unions  are  extremely  fre- 
quent, and  may  manifest  themselves  in  all  the  floral  verticils. 
Thus  the  sepals  may  become  soldered  together  and  form  a 
monsepalous  calyx,  or  the  petals,  a  monopetalous  corolla ;  and  in 
like  manner  the  stamens  may  unite  together  by  their  filaments 
and  become  monadelphous,  diadelphous,  or  polyadelphous,  or 
by  their  anthers  and  become  synantherous  or  syngenesious;  and 
lastly,  the  carpels  may  become  united  together  by  their  ovaries, 
or  by  their  ovaries,  styles  and  stigmas,  so  as  to  constitute  an 
apparently  unique  pistil.  These  different  kinds  of  soldering 
are  very  common  amongst  flowers,  and,  generally  speaking,  they 
do  not  alter  their  symmetry  and  regularity. 

But,  in  other  flowers,  it  is  more  difficult  to  perceive  at  first 
sight  what  it'is  that  disturbs  their  regularity  and  symmetry,  as 
for  instance  when  two  or  more  pieces  of  the  same  verticil 
become  soldered  together.  In  general,  however,  with  a  little 
care,  the  number  of  petals  or  sepals  which  have  united,  and 


MODIFICATIONS   OF   THE   FLORAL   ORGANS.  179 

the  nature  and  extent  of  the  soldering,  may  be  easily  detected. 
For  example,  if  the  number  of  petals  or  divisions  of  a  mono- 
petalous  corolla  do  not  correspond  to  those  of  the  calyx,  and 
this  difference  is  due  to  the  cohesion  of  one  or  more  of  the 
petals,  the  nature  of  the  soldering  may  be  readily  detected  by 
the  number  of  the  midribs.  Thus  when  two  petals  have  united 
in  the  place  of  one  nerve,  we  shall  detect  two  collateral  nerves 
in  the  petal,  or  three,  one  of  which  will  be  in  the  middle  when 
the  compound  petal  results  from  the  union  of  three  petals.  In 
general,  the  number  of  midribs  in  the  compound  petal  or  sepal 
will  be  sufficient  to  show  the  number  of  separate  pieces  which 
have  become  soldered  together. 

Let  us  take  for  illustration  and  analysis,  the  flower  of  the 
common  Snap-dragon  (Linaria,)  (Fig.  81.)     The  calyx  is  mo- 
Fig.  81. 


nosepalous,  and  has  five  equal  divisions.  The  corolla  is  mono- 
petalous  with  two  unequal  lips,  of  which  the  superior  repre- 
sents two  petals,  the  inferior,  three,  whose  midrib  is  prolonged 
into  a  spur.  The  stamens  are  four  in  number,  two  long,  and 
two  short;  the  former  being  situated  between  the  middle  petal 
and  the  two  lateral  petals  of  the  lower  lip,  the  latter  being 
placed  in  the  fissures  which  separate  the  two  lips.  At  the  base 
of  the  superior  lip  may  be  detected  a  little  filament  represent- 
ing the  fifth  stamen. 


180          COMPOUND  ORGANS  OF  PLANTS. 

In  certain  circumstances  the  Linarias  develope  themselves 
with  all  their  petals  similar  to  the  middle  petal  of  the  lower 
lip,  and  the  verticil  presents  then  a  perfectly  regular  figure. 
It  is  a  corolla,  with  five  lobes  and  five  spurs,  perfectly  equal 
among  themselves.  At  the  same  time,  the  filament  placed  at 
the  base  of  the  superior  lip  developes  itself  into  a  stamen 
organized  like  the  others,  which,  although  unequal  in  their 
habitual  condition,  are  now  absolutely  the  same  in  length,  after 
the  manner  of  a  flower  provided  with  five  symmetrical  stamens. 

The  name  peloria  (rttxwpwj  monstrous,)  has  been  given  to 
this  kind  of  metamorphosis ;  but  modern  botanists,  far  from 
regarding  this  change  as  a  digression  of  nature,  consider  it  as 
a  return  to  the  normal  state  of  the  flower.  To  their  eyes,  an 
irregular  flower  is  an  habitual  alteration,  and  a  pelorious  flower 
is  a  flower  put  into  regular  order. 

The  parts  or  organs  of  the  same  verticil  may  have  been  un- 
equally developed.  This  inequality  of  development  is  strikingly 
shown  in  the  papilionaceous  corolla  of  the  pea,  the  parts  of 
which  are  distinguished  by  separate  names.  This  plant  has  all 
the  parts  of  a  symmetrical  pentamerous  calyx  and  corolla,  only 
they  are  irregular  on  account  of  an  inequality  in  their  develop- 
ment. In  certain  orders  of  the  papilionacea  the  corolla  has, 
however,  a  tendency  to  become  regular,  and  in  Cassia,  the  five 
petals  differ  very  little  from  each  other  either  in  shape  or  size. 

One  or  more  Jloral  verticils  may  have  united  with  each  other. 
Thus  the  stamens  are  united  to  the  calyx  in  the  rose  and  black- 
berry, and  to  all  monopetalous  corollas.  So  also  the  calyx  is 
often  united  to  the  ovary  as  in  the  apple,  in  which  case  the 
sepals,  petals,  stamens  and  pistils  appear  to  grow  out  of  its 
summit,  and  the  ovary  is  said  to  be  inferior,  as  in  the  honey- 
suckle and  dog-wood.  More  rarely,  the  two  interior  verticils, 


MODIFICATIONS  OP  THE  FLORAL  ORGANS.      181 

the  stamens  and  carpels,  cohere  together.  This  case,  neverthe- 
less, presents  itself  in  Orchideous  plants,  which  constitute  the 
true  gynandrous  plants  of  Linnaeus. 

The  adherence  of  the  different  verticils  among  themselves  is 
called  their  insertion.  It  is  above  all  essential  to  study  the 
insertion  of  the  stamens,  as  it  furnishes  for  the  natural  co-ordi- 
nation of  plants,  characters  of  the  first  value.  Three  modes  of 
insertion  have  been  distinguished,  called  hypogynous  p*o 
under,  ywy  female  or  pistil),  perigynous  (  rtspi  around),  and 
epigynous  (irti  upon.) 

The  hypogynical  insertion  is  that  in  which  the  stamens  are 
inserted  upon  the  ovary,  which  is  therefore  necessarily  free  and 
superior,  as  for  instance,  in  the  Ranunculus.  This  kind  of  in- 
sertion is  readily  recognized  in  this,  that  we  are  able  to  remove 
the  calyx  without  carrying  the  stamens  away  at  the  same  time. 

The  perigynical  insertion  takes  place  when  the  stamens  are 
attached  to  the  calyx  and  surround  the  ovary,  as  in  the 
strawberry  (Fragaria.)  This  is  distinguished  by  this,  that 
when  we  remove  the  calyx,  we  necessarily  remove  the  stamens 
at  the  same  time,  which  are  inserted  on  it. 

The  epigynical  insertion  is  that  in  which  the  stamens  are 
inserted  upon  the  superior  part  of  the  ovary,  which  necessarily 
happens  whenever  the  ovary  is  inferior. 

One  or  more  organs  of  the  same  verticil,  may  have  been 
suppressed  or  rendered  abortive.  Abortions  and  suppressions 
contribute  more  than  any  other  cause  to  destroy  the  symmetry 
and  regularity  of  the  floral  organs.  Abortion  is  the  state  of 
an  organ  which,  after  having  commenced  to  form,  is  arrested  in 
its  evolution  and  remains  reduced  to  a  species  of  stump,  some- 
times a  gland  ;  suppression  indicates  the  total  absence  of  the 
organ,  which  has  not  even  commenced  to  develope  itself. 

16* 


182          COMPOUND  ORGANS  OP  PLANTS. 

The  symmetry  of  the  flower  is  frequently  destroyed  by  the 
abortion  of  one  or  more  organs  of  the  same  verticil.  In  the 
natural  order  Scrophulariacese  we  are  able  to  follow,  step  by 
step,  the  progressive  abortion  and  final  suppression  of  an  organ1, 
as  for  instance,  a  stamen,  by  examining  the  flowers  of  its  dif- 
ferent genera.  Thus  if  we  look  at  the  flower  of  the  common 
mullein  (Verbascum)  which  is  placed  at  the  head  of  the  order, 
we  shall  find  that  it  is  symmetrical  and  pentamerous  although 
somewhat  irregular  in  its  construction,  having  a  calyx  of  five 
sepals,  a  corolla  of  five  petals  soldered  together,  the  lobes 
broad,  rounded,  and  a  little  unequal ;  the  stamens  are  five,  and 
alternate  with  the  lobes  of  the  corolla ;  but  one  of  the  stamens 
is  a  great  deal  less  than  the  others ;  it  has  proved  already  a 
certain  degree  of  arrest  in  its  development.  In  Pentstemon 
the  anther  is  abortive,  and  the  stamen  appears  in  the  form  of 
a  bearded  filament.  In  Linaria  it  may  be  detected  in  the 
form  of  a  little  filament  at  the  base  of  the  superior  lip  of  the 
corolla.  If  now  we  examine  a  flower  of  the  Scrophularia  we 
shall  observe  no  more  than  four  stamens.  However,  between 
the  two  upper  lobes  of  the  corolla,  on  its  interior  surface  we 
shall  find  a  little  glandular  scale,  occupying  the  very  place  of 
the  missing  stamen,  and  of  which  it  is  not  difficult  to  recognise 
the  nature  Lastly,  if  we  open  a  flower  of  the  Digitalis  or 
Antirrhinum,  we  shall  find  no  trace  of  the  fifth  stamen,  which 
has  completely  disappeared. 

The  abortion  and  suppression  of  the  staminal  verticil  is 
carried  still  further  in  other  genera  of  the  same  family.  Thus 
Gratiola  Virginica  has  a  calyx  of  five  sepals,  five  petals  united 
almost  to  their  tips,  and  only  two  perfect  stamens,  the  three 
others  having  been  entirely  suppressed.  But  we  can  satisfy 
ourselves  that  this  abortion  and  ultimate  suppression  of  the 


MODIFICATIONS   OF   THE   FLORAL   ORGANS.  183 

organs  has  been  gradual,  for  in  another  species  of  the  same 
genus,  the  Gratiola  aurea,  two  minute  filaments  occupy  the 
place  of  two  of  the  stamens,  although  no  trace  of  the  fifth  is 
observable.  In  Gerardia  a  pair  of  perfect  stamens  a  little 
shorter  than  the  other  two,  occupy  the  place  of  these  filaments, 
and  the  stamens  are  thus  rendered  didynarnous. 

In  the  Labiat83  or  mint  tribe,  which  are  all  pentamerous 
flowers,  the  fifth  stamen  is  suppressed  altogether,  and  the  didy- 
namous  form  prevails ;  or  we  have  two  long  and  two  short 
stamens,  the  filaments  of  each  pair  being  unequally  developed. 
That  didynamous  stamens  are  the  consequence  of  a  defective 
development  of  the  organs,  is  evident  from  the  fact  that  in 
other  genera  of  the  same  family  the  development  of  the  organs 
has  been  arrested  at  an  earlier  stage,  so  that  the  two  short 
stamens  are  either  reduced  to  mere  filaments,  or  are  absent 
altogether  from  the  corolla,  the  npwer  being  strictly  speaking 
diandrous. 

The  symmetry  and  regularity  of  the  floral  organs  is  more 
frequently  disturbed  by  the  defective  development  or  entire 
suppression  of  one  or  more  of  the  organs  of  a  verticil,  than  by 
any  other  cause.  Such  deviations  from  the  normal  structure 
are  very  common,  in  fact  almost  any  flower  will  discover  them 
to  the  intelligent  student,  and  the  principle  when  once  clearly 
understood  may  be  extended  almost  indefinitely. 

We  close  our  illustrations  of  this  topic  with  the  following 
analysis  of  the  Spring  beauty  (Claytonia),  a  very  common  but 
remarkably  unsymmetrical  flower.  (Fig.  82)  is  a  diagram  show- 
ing the  flower  and  its  ground  plan.  It  will  be  perceived  that 
the  flower  is  complete,  for  the  four  verticils  are  present,  some 
of  them  being  partially  but  not  entirely  suppressed ;  that  the 
flower  is  regular,  for  all  the  pieces  of  each  floral  verticil  are 


184          COMPOUND  ORGANS  OP  PLANTS. 

Fig.  82. 


equally  developed,  but  that  it  is  remarkably  unsymmetrical,  for 
only  two  of  the  four  circles  have  the  same  number  of  members, 
and  one  of  them,  viz.,  the  stamina te  circle  is  in  an  abnormal 
position ;  for  instead  of  alternating  with  the  petals,  we  find  the 
stamens  in  this  flower  placed  directly  opposite  the  petals. 
The  diagram  shows  a  calyx  with  two  sepals,  but  as  the  normal 
construction  of  the  flower  is  evidently  pentamerous,  three  of 
the  sepals  have  been  suppressed,  and.  the  two  which  have  been 
developed,  have  evidently  obtained  possession  of  the  place 
which  the  three  by  their  absence  had  left  void.  Within  the 
calycine  verticil,  are  five  equally  developed  petals  alternating 
with  $he  sepals  of  the  calyx,  and  which  are  therefore  regular 
and  normal  in  their  growth.  Within  and  opposite  the  petals 
we  find  five  stamens.  The  number  of  the  stamens  is  normal, 
but  not  their  position.  Here  is  an  evident  departure  from 
that  law  of  alternation  which  usually  manifests  itself  in  the 
relative  position  of  the  pieces  of  the  floral  verticils,  when  they 
follow  each  other  directly  in  the  flower. 


MODIFICATIONS   OP   THE   FLORAL   ORGANS.  185 

It  would  appear  from  this,  that  a  verticil  of  stamens  has 
been  suppressed,  and  these  stamens  belong  fro  a  second  verticil 
and  are  therefore  necessarily  opposite  the  petals.  It  is  true 
that  the  abortive  stamens  have  left  no  traces  of  their  existence 
within  the  corolla,  but  this  does  not  disprove  this  method  of 
accounting  for  their  absence,  because  it  receives  abundant 
confirmation  from  what  we  see  in  other  orders  of  plants  which 
are  equally  defective  in  the  stamineal  cirde.  Thus .  in  the 
natural  order  Priinulaceae,  the  general  disposition  of  the  floral 
organs  is  as  follows.  A  monosepalous  calyx  of  five  sepals,  a 
monopetalous  corolla  of  five  petals,  the  lobes  of  which  are 
situate  opposite  the  sinuses  of  the  calycine  lobes,  and  therefore 
in  their  normal  position ;  and  a  verticil  of  five  stamens  which 
are  directly  opposite  the  petals.  The  law  of  alternation  is 
'therefore  defective  in  the  staminical  circle.  We  account  for 
the  phenomena  in  the  same  manner  as  we  explain  it  in  Clay- 
tonia,  on  the  theoretical  supposition  that  the  primary  alter- 
nating verticil  of  stamens  is  generally  suppressed  in  this  family. 
We  examine  the  other  genera  of  this  family  for  confirmation 
of  our  theory,  and  we  find,  that  although  the  primary  verticil 
of  stamens  is  suppressed  in  most  of  the  corollas  of  the  order, 
yet  this  very  verticil  is  present  in  some  of  them  in  an  inter- 
mediate stage  of  development.  Thus  in  Samolus  floribundus, 
for  example,  there  are  found  in  the  sinuses  of  the  corolla,  in 
the  normal  position  of  the  absent  verticil,  five  sterile  filaments 
which  are  unquestionably  these  very  stamens  in  a  rudimentary 
condition.  When,  therefore,  we  find  in  the  Claytonia  and 
other  plants,  a  verticil  of  stamens  opposite  the  petals  of  the 
corolla,  instead  of  alternating  with  those  petals,  we  are  justified 
in  supposing  that  the  primary  alternating  verticil  has  been 
suppressed,  although  we  may  not  be  able  to  recognize  any 
traces  of  its  existence  in  the  corolla. 


186  COMPOUND   ORGANS   OP    PLANTS. 

Lastly,  in  the  centre  of  the  flower  of  the  Claytonia,  there 
are  three  pistils ;  two  have  therefore  been  suppressed. 

One  or  more  entire  verticils  may  have  been  wholly  sup- 
pressed. We  have  already  said  that  the  complete  flower 
consists  of  four  verticils  of  metamorphosed  leaves,  viz.,  the 
calyx,  corolla,  stamens  and  pistils :  now  if  any  one  of  these 
verticils  be  absent,  the  flower  is  incomplete.  Deviations 
resulting  from  the  non-production  of  the  verticils  are  not 
uncommon,  and  may  affect  any  of  the  floral  organs.  Thus  the 
calyx  is  reduced  to  an  obscure  ring  or  border  in  the  holly  and 
dogwood,  and  is  suppressed  altogether  in  the  prickly  ash, 
(Zanthoxyllum.)  We  infer  that  it  is  the  corolla  which  remains 
in  this  flower  because  the  five  stamens  alternate  with  it.  In 
other  instances  the  corolla  is  suppressed  and  the  calyx  remains 
as  in  Anemone  and  Clematis. 

It  is  proper  to  remark  here,  that  where  there  is  only  one 
floral  envelope,  the  law  of  alternation  will  enable  us  to  detect 
whether  it  is  a  calyx  or  a  corolla.  Thus  if  the  corolla  is  sup- 
pressed, it  is  easy  to  see  that  the  two  verticils  between  which 
the  corolla  is  developed  will  have  their  parts  opposite,  that  is 
to  say,  the  androecium  and  the  calyx.  This  is  their  natural  posi- 
tion, since  the  stamens  alternating  with  the  petals  are  necessa- 
rily placed  opposite  to  the  sepals ;  and  therefore  when  we  find 
them  in  this  position  and  only  one  floral  envelope,  we  may  con- 
clude that  envelope  to  be  the  calyx  and  the  petals,  to  have  been 
suppressed,  whatever  may  be  its  color  and  hue.  If,  on  the 
contrary,  there  is  a  single  envelope  with  which  the  stamens 
alternate,  we  may  conclude  that  it  is  the  calyx  which  has  been 
suppressed,  and  that  the  colored  envelope  is  a  true  corolla. 
This  appears  to  be  an  easy  way  of  settling  this  question  where 
other  methods  fail. 


MODIFICATIONS   OF   THE   FLORAL   ORGANS.  187 

Not  unfrequently,  however,  the  floral  envelope  which  has 
disappeared,  has  left  traces  of  its  existence  sufficient  to  disclose 
the  character  of  that  which  remains.  We  know  that  the  pro- 
per place  of  the  petals  is  between  the  sepals  and  the  stamens, 
and  if  we  find  anything  occupying  their  position,  we  are  at 
once  convinced  from  the  laws  of  development  which  are  so 
clearly  and  beautifully  expressed  in  other  genera,  that  it  is 
the  same  organ  in  an  abortive  or  rudimentary  condition.  Thus 
the  flower  of  Pulsatilla  patens  has  only  one  floral  envelope. 
In  the  place,  however,  usually  occupied  by  the  petals,  we  find 
certain  abortive  gland-like  stamens,  which  are  in  fact  the  rudi- 
ments of  the  suppressed  petals;  this  therefore  decides  the 
envelope  to  be  a  calyx. 

In  some  plants,  such  as  nettles  and  Chenopodiums,  the 
floral  envelopes  are  green  and  inconspicuous,  and  in  the  grasses 
they  are  suppressed  altogether,  their  places  being  supplied  by 
rudimentary  leaves  or  bracts.  When  this  is  the  case,  the  flower 
ceases  to  attract  popular  attention.  The  world  attaches  the 
idea  of  a  flower  to  that  part  of  a  plant  which  is  usually  colored 
with  tints  more  or  less  brilliant,  which  makes  its  appearance 
generally  before  the  seed,  and  after  delighting  our  senses  with 
its  fragrance  and  beauty  for  a  brief  space  of  time,  is  replaced 
by  the  fruit  or  seed.  But  such  flowers  are  only  characteristic 
of  the  more  perfect  races  of  plants.  The  botanist  stops  not  at 
these  appearances,  for  to  him  the  flower  is  often  deprived  of 
them.  The  student  must  learn  to  recognize  the  flower  in  the 
lower  degrees  of  its  development. 

When  both  stamens  and  pistils  are  present  in  the  same 
flower,  it  is  said  to  be  hermaphrodite  and  complete.  When  on 
the  contrary,  the  flower  contains  stamens  only,  or  pistils  only, 
it  is  denominated  unisexual,  and  is  male  or  female  according 


188  COMPOUND   ORGANS   OP    PLANTS. 

as  the  former  or  the  latter  only  are  found  within  the  floral 
envelopes. 

When  the  stamens  and  pistils  are  in  separate  flowers  on  the 
same  plant,  as  in  the  castor-oil  plant,  (Ricinus,)  and  Indian 
corn,  (Zea  mays,)  the  flowers  are  monrecious,  (/tdi/oj  one,  otxto? 
habitation.)  When  the  staminate  flowers  are  on  one  plant  and  the 
pistillate  flowers  on  another,  the  flowers  are  dio3cious  (&$  twice,) 
as  in  the  nettle  and  hop ;  and  when  the  same  plant  developes 
both  unisexual  and  hermaphrodite  flowers,  they  are  polyga- 
mous (rtohvs  many,  ya^oj  marriage,)  as  in  the  maple  and  Euphor- 
bia. In  the  marginal  flowers  of  Hydrangea  arborescens,  and 
Viburnum  opulus,  the  Snow-ball  tree,  the  essential  organs,  the 
stamens  and  pistils,  are  entirely  suppressed ;  these  flowers  are 
therefore  necessarily  sterile. 

The  following  diagrams  will  illustrate  the  several  stages  in 
the  suppression  of  the  floral  organs  of  Phanerogamous  plants, 
until  we  arrive  at  their  minimum  reduction,  when  any  farther 
suppression  would  render  the  production  of  an  embryo  or  seed 
impossible. 

Fig.  83. 


Fig.  83,  is  a  representation  of  the  flower  of  the  Saururus 
cernuus  or  Lizard's  tail.  The  flowers  of  this  plant  are  perfect, 
and  are  developed  in  racemes  or  spikes,  but  destitute  of  all 
floral  envelopes,  a  simple  scale  or  bract  supplying  their  place. 


MODIFICATIONS   OF   THE   FLORAL   ORGANS.  189 

Fig.  84.  Fig.  85. 


This  plant  is  common  along  the  margins  of  streams  and  ponds, 
and  may  be  found  in  bloom  in  the  month  of  June. 

Fig.  84,  is  the  stinging  nettle,  (Urtica,)  bearing  clusters  of 
greenish  inconspicuous  unisexual  flowers  a,  in  the  axils  of  its 
leaves.  Fig  86,  one  of  the  male  flowers  magnified.  Its 
perianth  is  a  simple  calyx  of  four  sepals,  within  which  are  four 
stamens  opposite  to  the  sepals,  situated  on  the  receptacle,  and 

17 


190  COMPOUND  ORGANS  OF  PLANTS. 

this  relation  of  parts  leads  at  once  to  the  detection  of  the  fact 
that  the  corolla  has  been  suppressed.  The  perianth  of  the 
female  flower  is  also  a  simple  calyx,  consisting  of  four  very 
unequal  sepals,  the  two  outer  small,  the  inner  foliaceous, 
enclosing  a  single  pistil. 

Fig.  86.  Fig.  87. 


Fig.  85,  shows  a  compound  spike  of  wheat,  (Triticum,)  with 
numerous  spikelets  or  flowers  arranged  along  the  axis  in  a 
zigzag  form.  Fig.  87,  one  of  these  spikelets  magnified,  and 
deprived  of  its  glumes,  showing  the  three  stamens  a,  hanging 
by  long  thread-like  filaments,  and  the  feathery  styles  of  the 
pistil  within  two  bracts  sq.  The  flower  in  this  case  is  herma- 
phrodite. 

The  flower  is  therefore  still  further  reduced  in  the  sedges 
(Carices,)  which  are  equally  without  floral  envelopes,  and  are 
unisexual. 

Fig.  88,  shows  the  monoacious  flowers  of  a  species  of  Carex. 
a.  One  of  the  staminate  flowers,  consisting  of  a  single  glume 
or  scale  and  three  stamens,  b.  One  of  the  pistillate  flowers. 


MODIFICATIONS   OP   THE   FLORAL  ORGANS. 
Pig.  88. 


191 


This  pistil  is  covered  by  an  urceolate  glumaceous  bag  marked  u, 
called  a  perigynium.     There  is  one  style  sty  with  three  stigmas 

at  its  summit. 

Fig.  89. 


Fig.  89,  is  a  representation  of  a  common,  though  exceed- 
ingly interesting  aquatic  plant,  the  Callitriche  verna,  (xaxoj 
beautiful,  and  0p«|  hair,  in  allusion  to  its  capillary  and  tufted 
stems.)  The  lower  leaves  of  the  stem  are  immersed  and  linear, 
the  upper  floating  and  spatulate.  The  flowers  are  polygamous, 
unisexual  and  hermaphrodite  flowers  growing  together  on  the 
same  plant.  They  are  without  either  calyx  and  corolla,  have 


192          COMPOUND  ORGANS  OP  PLANTS. 

not  even  a  bract,  and  consist  of  a  single  stamen  and  pistil 
placed  together  in  the  axil  of  the  leaves,  when  hermaphrodite 
and  complete,  and  when  unisexual,  placed  apart  from  each 
other.  In  Fig.  89,  the  male  flower  consists  of  a  single  stamen 
and  the  female  flower  is  represented  by  a  solitary  pistil.  In 
the  Callitriche,  therefore,  the  flower  is  finally  reduced  to  a 
minimum. 


CHAPTER   XV. 

THE   FRUIT,   OR   MATURE   OVARY. 

THE  term  fruit,  as  understood  among  botanists,  has  a  more 
extended  signification  than  its  meaning  in  ordinary  language. 
It  is  applied  by  them  to  the  fecundated  and  mature  ovary 
enclosing  seeds,  capable  of  germinating  and  reproducing  the 
plant,  whatever  be  its  form  or  texture,  and  whether  it  be  edible 
or  not.  In  this  respect  a  grain  of  wheat  or  corn,  or  the  peri- 
carp of  the  sun-flower  or  thistle,  is  as  much  a  fruit  as  a  peach, 
gooseberry,  or  melon. 

Very  often,  besides  the  ovary,  other  parts  of  the  flower,  and 
especially  the  calyx,  enter  into  the  composition  of  the  fruit; 
but  these  are  only  accessory  parts,  the  term  fruit  being  strictly 
applicable  only  to  the  ovary. 

The  fruit  is  composed  of  two  parts,  the  pericarp  (rtepi,  around, 
xaprtoj  fruit,)  and  the  seed  or  seeds.  The  pericarp  is  formed  by 
the  walls  of  the  ovary  itself;  the  seeds  are  the  ovules  fecun- 
dated and  containing  an  embryo.  Let  us  consider  each  of 
these  parts  in  succession. 

THE  PERICARP. — The  pericarp  is  that  part  of  the  fruit 


THE   FRUIT.  193 

which  is  formed  by  the  walls  of  the  ovary,  and  which  deter- 
mines the  general  form  of  the  fruit.  Since  the  walls  of  the 
ovary  constitute  the  pericarp,  it  must  be  constantly  present  in 
all  fruits.  When  the  fruit  is  a  single  cell  and  contains  only  one 
seed,  the  pericarp  is  so  thin  and  is  united  so  completely  with 
the  seed,  that  they  can  hardly  be  distinguished  from  each 
other.  Such,  for  example,  are  the  fruits  of  the  grasses,  Cype- 
raceae,  and  syngenesious  plants,  which  were  formerly  regarded 
as  seeds,  but  which  are  in  reality  pericarps  or  seed-vessels 
enclosing  a  seed. 

A  fruit  may  be  usually  distinguished  from  a  seed,  or  other 
organ  assuming  its  character,  by  the  presence  of  some  vestige 
of  the  style.  Thus  the  carpels  of  the  Ranunculus,  (Fig.  90,) 

Fig.  90. 


Fig.  90.  Carpels  of  the  Ranunculus  with  a  few  stamens,  the  calyx  and  corolla  having 
been  removed.  One  of  the  carpels  magnified,  showing  it  to  be  a  single-seeded  vessel 
with  the  pericarp  applied  close  to  the  seed.  Such  fruits  resemble  seeds  in  appearance, 
the  style  and  stigma,  s,  aid  in  distinguishing  them  from  seeds. 

which  are  vulgarly  regarded  as  seeds,  are  at  once  determined 
to  be  seed-vessels  by  their  apiculate  summit,  the  vestige  of  the 
style.  In  the  same  manner  we  discover  that  the  strawberry  is 
not  a  single  fruit,  but  an  enlarged  fleshy  receptacle  bearing  the 
simple  fruits  at  its  surface.  (Fig.  91.) 

The  pericarp,  like  the   leaves  from  which  it  proceeds,  is 
composed  of  two  plates  of  epidermis,  between  which  exists  a 

17* 


194          COMPOUND  ORGANS  OP  PLANTS. 


Fig.  91. 


Fig.  91.  Fruit  of  strawberry,  (Fragaria  vesca,)  showing  the  carpels  or  achenia  on  the 
surface  of  its  enlarged  and  fleshy  receptacle.  Each  achenium  has  a  style  and  stigma, 
and  is  thus  at  once  distinguished  from  a  seed.  The  calyx  is  seen  at  the  base  of  the 
receptacle. 

cellulo- vascular  bed  of  fibres  and  parenchyma.  The  exterior 
membrane  of  the  pericarp  is  called  the  epicarp,  (Itti  upon, 
xaprtof  fruit,)  and  corresponds  to  the  lower  epidermis  of  the 
leaf.  This  membrane  is  ordinarily  very  thin,  and  is  easily 
removed,  especially  in  succulent  fruits,  such  as  the  peach  or 
plum.  The  interior  membrane  of  the  pericarp  immediately 
surrounding  the  seed,  is  called  the  endocarp,  (tvdov  within,) 
and  is  equivalent  to  the  upper  epidermis  of  the  leaf.  It  is 
usually  thin  and  membranaceous,  and  sometimes  appears  like 
parchment,  as  in  the  pea  and  apple.  In  the  peach  and  plum 
it  takes  a  ligneous  consistence,  and  forms  the  stone  or  puta- 
men,  (putamen  a  shell,)  immediately  investing  the  kernel  or 
seed  of  these  fruits.  The  intermediate  tissue  of  the  pericarp 
between  the  epicarp  and  the  endocarp,  which  represents  the 
parenchyma  of  the  leaf,  is  called  the  mesocarp,  (ptaos  middle.) 
The  mesocarp  is  more  or  less  succulent,  according  to  the  pro- 
portionate development  of  its  two  constituents,  fibres  and 
parenchyma.  It  is  very  much  developed  in  fleshy  fruits, 
forming  their  flesh  or  pulp,  as  in  the  peach  and  plum,  and 
hence  it  has  been  sometimes  called  the  sarcocarp,  (<yapf  flesh.) 


THE   FRUIT.  195 

But  sometimes  the  mesocarp  is  excessively  thin,  in  dry  fruits 
for  example,  such  as  the  pod  of  the  pea,  or  the  fruit  of  the 
gilliflower.  In  the  nut  the  three  parts  are  blended  together ; 
in  the  peach  they  remain  separate.  In  the  latter  fruit  the 
epicarp  forms  the  skin,  the  mesocarp  the  fruit  or  edible  part 
of  the  peach,  and  the  endocarp,  the  stone  in  its  centre  which 
covers  the  kernel  and  seed. 

Whatever  may  be  the  thickness  of  the  walls  of  the  pericarp, 
its  anatomical  constitution  remains  the  same.  It  is  always 
formed  of  two  membranes,  the  epicarp  and  the  endocarp,  and 
an  intermediate  bed  of  tissue  called  the  mesocarp,  sometimes 
thin  and  dry,  at  other  times  thick  and  succulent.  Such  is  the 
constitution  of  the  leaf  from  which  it  is  derived,  and  of  which 
it  is  only  a  peculiar  modification. 

This  remarkable  transformation  of  the  leaves  is  not  peculiar 
to  fruits,  for  in  more  cases  than  is  usually  supposed,  similar 
changes  take  place  in  the  other  floral  organs.  Thus  the  calyx  is 
changed  into  a  hard  crustaceous  body  in  Salsola  and  in  Spinage; 
and  is  red  and  juicy  in  the  Strawberry  blite  and  Winter- 
green  (Gaultheria),  being  in  both  instances  commonly  mistaken 
for  the  fruit  from  which  it  is  wholly  distinct.  In  the  Yew, 
the  bracts  enveloping  the  seed  become  pulpy  and  berry-like. 
Nearly  the  whole  bulk  of  the  apple  is  a  thickened  calyx.  The 
pulp  of  the  Strawberry,  as  we  have  already  intimated,  is 
nothing  else  but  the  enlarged  and  juicy  extremity  of  the 
1  flower-stalk  or  receptacle.  Examples  might  be  multiplied 
proving  that  all  the  appendages  of  the  axophyte  are  subject  to 
these  transformations,  which  are  erroneously  imagined  to  be 
peculiar  to  the  fruit. 

The  fruit,  like  the  pistil  of  which  it  is  the  final  development, 
may  be  either  simple  or  compound.  The  fruit  is  simple  when 


COMPOUND  ORGANS  OP  PLANTS. 

it  proceeds  from  a  simple  carpel  or  pistil.  In  this  case  the 
pericarp  presents  constantly  one  single  cell,  or  it  is  unilocular. 
The  compound  fruit  proceeds  from  the  compound  pistil,  the 
pericarp,  like  the  ovary,  containing  as  many  cells  as  the  number 
of  pistils  which  have  united.  Thus  the  pericarp  is  bi-locular 
in  the  tobacco,  tri-locular  in  the  tulip,  quadri-locular  in  the 
epilobium,  quinque-locular  in  flax,  &c.  We  have  already  made 
known  these  particulars  in  treating  of  carpels. 

It  is  necessary  to  observe  here,  that  the  number  of  cells  in 
the  pericarp  or  ripe  fruit,  does  not  always  exactly  correspond 
to  the  structure  of  the  ovary.  It  often  happens,  between  the 
moment  of  fecundation  and  the  maturity  of  the  seed,  that  con- 
siderable changes  take  place  in  the  internal  structure  of  the 
ovary,  a  number  of  its  dissepiments  being  absorbed  during  its 
progress  towards  maturity,  so  that  an  ovary  originally  multilo- 
cular  becomes  finally  an  unilocular  fruit  or  pericarp.  A  great 
many  of  the  Caryophyllaceae  and  the  Cistaceae  are  in  this  case. 
The  rapid  increase  of  their  ovaries  break  and  efface  their  disse- 
piments, so  that  they  are  not  found  in  the  mature  pericarp. 
Alterations  take  place,  not  only  in  the  number  of  cells,  but 
also  in  the  seeds,  the  ovules  being  equally  liable  to  become 
obliterated.  In  the  acorn,  (Fig.  92,)  the  young  pistil  is  formed 
of  three  carpels,  the  ovary  consisting  of  three  cells  with  two 
ovules  in  each  cell  as  represented  in  the  transverse  section. 
But  the  walls  of  the  cells  and  five  of  the  ovules  are  suppressed  in 
the  progress  of  development,  so  that  the  pericarp  ultimately 
becomes  'unilocular  and  monospermal,  or  one-seeded.  Hence 
the  acorn  or  fruit  of  the  oak  (Quercus),  consists  of  a  one-seeded 
pericarp,  surrounded  by  an  involucre  of  bracts  forming  the  cup 
or  cupula. 

Dehiscence  of  the  pericarp.     When  the  fruit  has  arrived  at  a 


THE   FRUIT.  197 

Fig.  92. 


state  of  maturity,  the  pericarp  opens  to  let  the  seeds  escape. 
The  fruits  which  open  spontaneously  in  this  manner  are  said  to 
be  dehiscent,  (dehisco  I  gape).  However,  there  are  some  fruits 
which  fall  to  the  ground  without  opening  or  dehiscing.  The 
fleshy  pericarps  of  the  peach  and  apple  for  example,  do  not 
open ;  their  seeds  are  liberated  as  the  fruit  decays.  The  dry 
pericarps  of  the  Compositse,  the  Maize,  and  the  Ranunculus, 
remain  indehiscent  on  the  soil,  enveloping  the  grain  till  the 
plantule  in  germinating  forces  a  passage  through  them. 

The  pericarp,  whether  it  proceeds  from  a  single  pistil,  or 
from  one  that  is  compound,  always  presents  on  its  outer  surface 
longitudinal  lines  which  are  called  sutures.  One  of  these 
sutures,  formed  by  the  union  of  the  free  margin  of  each  carpel- 
lary  leaf,  is  called  the  ventral  suture ;  the  other,  exactly  oppo- 
site, and  corresponding  to  its  midrib,  is  named  the  dorsal 
suture;  the  former  is  generally  connected  with  the  axis,  the 
latter  with  the  periphery  of  the  fruit.  In  a  simple  pericarp, 
such  as  the  pod  of  the  pea,  for  example,  both  these  sutures  are 
equally  visible  on  the  exterior  of  the  fruit.  But  when  the 
carpels  solder  together  by  their  lateral  surfaces,  and  form  a 


COMPOUND  ORGANS  OF  PLANTS. 

compound  pistil,  the  ventral  sutures  are  all  united  in  the  centre 
of  the  fruit,  and  we  see  on  the  exterior  only  the  dorsal  sutures. 
From  this  union  of  the  carpels  among  themselves  it  follows, 
that  new  lines  will  be  formed  at  their  points  of  contact.  These 
new  lines  called  parietal  sutures,  are  ordinarily  seen  on  the 
exterior  of  the  compound  ovary  between  the  dorsal  sutures, 
and  indicate  the  points  where  the  walls  of  the  several  carpel- 
lary  leaves  are  joined.  Finally,  when  the  carpellary  leaves, 
instead  of  folding  on  themselves,  uniting  by  their  free  margins 
and  soldering  by  their  lateral  surfaces,  so  as  to  cause  their 
ventral  sutures  to  meet  in  the  centre  of  the  pericarp,  and  each 
folded  carpellary  leaf  to  form  a  distinct  cell  in  its  cavity;  unite 
together  by  their  margins,  making  only  a  slight  introflexion 
towards  the  axis  of  the  pericarp,  in  such  a  manner  as  to  form 
a  unilocular  pericarp;  the  lines  which  result  from  this  union 
are  called  marginal  sutures.  The  nature  and  origin  of  these 
different  sutures  being  understood  by  the  student,  he  will  find 
no  difficulty  in  comprehending  the  several  varieties  of  valvular 
dehiscence. 

Dehiscence  usually  takes  place  in  simple  fruits  either  by  the 
ventral  or  dorsal  suture,  or  by  both.  Dehiscence  takes  place 
by  the  ventral  suture  in  the  poeony  and  Wild  Columbine 
(Aquilegia) ;  by  the  dorsal  suture  in  the  Magnolia ;  and  by 
both  sutures  in  the  pea  and  Acacia. 

When  the  fruit  consists  of  several  united  carpels  or  is  com- 
pound, the  dehiscence  may  take  place  through  the  parietal 
sutures  so  as  to  resolve  the  fruit  into  its  original  carpels,  as  in 
the  Colchicum,  (Fig.  96,)  when  it  is  septicidal  (septum  a  wall, 
and  ccedo  I  cut).  This  happens  when  the  lamina  of  the  car- 
pellary leaves  are  only  slightly  united.  When,  however,  these 
lamina  are  firmly  soldered  together  dehiscence  takes  place  by 


THE   FRUIT.  199 

Fig.  93.  Fig.  94.  Fig.  95. 


Fig.  93.  The  five  carpellary  leaves  or  follicles  of  Aquilegia,  opening  by  their  ventral 
suture. 

Jig.  94.  The  carpels  of  Magnolia  glauca  with  their  dorsal  sutures  open  and  the 
seeds  suspended  from  them  by  curious  extensile  cords. 

Fig.  95.  The  legumen  or  pod  of  the  pea,  opening  by  both  sutures  at  the  same  time. 
In  the  former  instances  the  fruit  was  univalve,  in  this  case  it  is  bivalve,  ep.  Epicarp. 
en.  Endocarp.  ov.  Ovules  attached  to  the  placenta  pi,  by  means  of  the  funiculus 
f.  The  legume  opens  by  both  ventral  and  dorsal  suture.  The  placenta  pi  is  double, 
and  runs  along  each  edge  of  the  ventral  suture.  At  the  apex  of  the  pod  are  seen 
the  remains  of  the  style  and  stigma,  and  at  its  base  the  remains  of  the  calyx. 

the  dorsal  suture,  and  the  several  lamina  are  detached  from 
their  midribs.  The  result  of  this  is,  that  each  of  the  valves 
carries  on  the  middle  of  its  internal  surface  a  double  lamina  or 
dissepiment,  which  is  composed  of  a  portion  of  the  united 
laminae  of  the  two  different  carpels,  as  in  the  Martagon  lily. 
(Fig.  97.)  This  dehiscence  is  loculicidal  (loculus  a  cell,  and 
ccedo  I  cut.) 

In  septicidal  dehiscence  each  valve  is  a  complete  carpel, 
and  generally  contains  the  ovules  attached  to  the  placenta. 
Tn  loculicidal  dehiscence,  however,  sometimes  the  placenta 
accompany  the  dissepiments,  as  in  the  Pansy.  Frequently, 


200 


COMPOUND   ORGANS   OF    PLANTS. 
Fig.  96.  Fig.  97. 


Fig.  97.  Dehiscence  of  the  three-celled  fruit  of  the  Martagon  lily,  showing  the  dis- 
sepiments in  the  middle  of  the  valves. 

however,  the  placenta  and  ovules  remain  firmly  attached 
together,  so  that  the  dissepiments  or  united  laminae  of  the 
several  carpellary  leaves  separate  from  their  margins  instead 
of  midrib,  which  margins  remain  united  and  persistent  in  the 
centre  of  he  pericarp,  forming  a  sort  of  central  axis  or  colu- 
mella,  as  in  the  morning-glory  (Convolvulus).  Lastly,  not  only 
the  margin  but  a  part  of  the  laminae  may  be  persistent  about 
the  central  axis,  so  that  when  the  pericarp  opens  at  the  parie- 
tal suture,  the  central  column  presents  as  many  walls  attached 
to  it  as  there  were  dissepiments  in  the  ovary  before  its  dehis- 
cence.  *  We  call  this  variety  of  loculicidal  dehiscence,  septifra- 
gal  (septum  and/rango  I  break). 

The  sutures  or  seams  of  the  pericarp,  instead  of  dehiscing  or 
splitting  through  their  entire  length,  are  sometimes  only  rup- 
tured for  a  short  distance  from  the  apex  as  in  the  chickweeds, 


THE   FRUIT.  201 

(Cerastium.)  In  the  snap-dragon,  (Antirrhinum),  the  sutural 
rupture  is  so  slight  as  to  produce  only  points  or  pores  in  the 
upper  part  of  the  pericarp. 

Fig.  98. 


Besides  these  regular  forms  of  valvular  dehiscence  there  is  a 
somewhat  anomalous  mode  of  rupture  which  takes  place  in  a 
few  plants,  such  for  instance  as  the  pimpernell  (Anagallis),  and 
henbane  (Hyoscyamus),  and  which  is  called  circurnscissile, 
(circum  around,  scindo  to  cut.)  The  pericarp  of  these  plants 
opens  by  a  transverse  circular  line,  following  no  sutures  what- 
ever but  cutting  directly  across  them.  It  is  therefore  an 
anomaly  and  not  a  true  dehiscence,  as  we  have  employed  the 
term.  Fig.  98  is  the  seed  vessel  of  the  Hyoscyamus  which  is 
ruptured  in  this  manner.  The  upper  part  of  the  pericarp 
separates  like  a  lid  from  the  lower  part.  This  kind  of  fruit  is 
called  a  pyxidium,  (pyxis  a  chest.) 

The  pods  of  some  Leguminous  plants  formed  by  a  single 
carpel,  are  divided  into  several  cells,  either  by  the  formation 
of  false  horizontal  partitions,  as  in  some  Cassias,  or  by  the 
contractions  of  the  legume  itself,  as  in  Desmodium.  Each  of 
these  cells  contains  a  separate  seed,  and  the  pod  when  ripe 
separates  by  transverse  dehiscence  at  these  joints,  and  falls  info 
pieces.  This  kind  of  pod  is  called  a  loment,  (Fig.  99).  This 

18 


202  COMPOUND  ORGANS  OP  PLANTS. 

Fig.  99. 


Fig.  99.    Loment  of  saintfoin  (Hedysarum) ,  which  separates  transversely  into  single 
seeded  portions. 

transverse  disarticulation  may  be  supposed  to  have  some 
relation  to  a  simply  pinnate  leaf,  whose  modification  in  this 
instance  forms  the  carpel,  the  divisions  indicating  the  points 
where  the  different  pairs  of  pinnae  have  United. 

Different  kinds  of  pericarps  or  fruits. — Several  eminent 
botanists  have  attempted  to  make  a  classification  of  the  dif- 
ferent kinds  of  pericarps.  We  have  not  space  for  the  enumera- 
tion of  any  more  than  those  which  most  frequently  occur,  and 
to  which  reference  is  most  generally  made.  The  principal 
indehiscent  fruits  or  pericarps  are, 

1.  The  Garyopsis  or  grain,  (xapva  a  nut,  and  04/15  appearance.) 
This  is  a  dry  indehiscent  one  seeded  pericarp,  which  is  so 
incorporated  with  the  seed  as  to  be  inseparable  from  it.     It 
is  seen  in  the  cultivated  grains  such  as  maize,  barley,  oats, 
which  in  common  language  are  called  seeds,  but  which  con- 
sidered botanically  are  not  seeds,  but  seed  vessels  containing  a 
seed.     It  is  only  by  examining  them  in  their  early  state  and 
noticing  their  styles,  that  we  can  become  convinced  that  these 
grains  are  only  apparent  but  not  real  seeds. 

2.  The  Achenium  (a  without,  and  xcw'fco  I  open),  is  a  single 
seeded  indehiscent  fruit,  the  pericarp  of  which  is  distinct  from 
the  coats  of  the  seed.     The  fruit  of  the  Ranunculus  consists  of 
a  number  of  these  achenia  borne  on  a  convex  receptacle.     In 
the  rose,  the  receptacle  which  supports  them  is  concave  and  is 


'THE  FRUIT.  203 

invested  by  the  swollen  and  succulent  fruit-like  calyx.  The 
fruits  of  tne  dandelion,  sun-flower,  and  all  Compositse,  are 
achenia  or  single-seeded  pericarps.  Each  has  been  produced 
by  a  separate  flower,  and  is  provided  with  a  persistent  calyx 
the  tube  of  which  is  closely  united  to  the  fruit,  its  limb  form- 
ing a  beautiful  stellate  down  at  the  summit  of  the  style  or 
ovary,  by  means  of  which  the  achenium  or  mature  ovary  is 
lifted  from  off  the  surface  of  the  broad  and  dilated  receptacle 
and  wafted  by  the  winds  to  spots  favorable  to  its  germination. 
The  bottom  of  the  persistent  calyces  of  the  Labiatae  or  mint 
family  usually  contain  four  achenia  which  look  at  first  like 
seeds,  and  were  actually  regarded  by  Linnaeus  as  such.  He 
defines  them  as  "semen  tectum  epidermide  ossea,"  that  is 
seed  covered  with  an  osseous  epidermis,  and  hence  he  called 
the  whole  order  gymnospermia,  (yvpvo$  naked,  <jrtfp^ta  a  seed.) 
The  student  may  however  easily  satisfy  himself  that  such  is 
not  the  case,  and  ascertain  that  they  are  pericarps  or  seed 
vessels  by  cutting  across  them,  when  he  will  discover  the  true 
seed  in  their  interior. 

The  Cremocarp. — This  fruit  is  confined  to  the  great 
natural  order  Umbelliferse,  of  which  the  carrot  and  parsley 
are  familiar  examples.  The  Cremocarp  (xpcftcuo,  to  suspend,) 
is  composed  of  two  achenia,  which  are  at  first  united  to  a 
common  axis  called  the  carpophore,  (xaprto?,  fruit,  and  $op£«,  I 
bear,)  which  axis  separates  at  maturity,  as  in  Fig.  100,  the 
two  achenia  being  placed  apart  and  suspended  from  its  summit. 
Each  of  these  achenia  is  called  a  mericarp  (n*epo$,  part,)  or 
hemicarp,  (q/uav?,  half,  and  xaprtbs,  fruit.) 

The  Samara,  (samera,  seed  of  elm.)  This  is  an  achenium 
with  a  membranaceous  appendage  attached  to  its  summit  or 
margin,  and  forms  those  peculiar  winged  fruits  suspended  in 


204          COMPOUND  ORGANS  OF  PLANTS. 
Fig.  100. 


Fig.  100.  Cremocarp  of  fennel  (Foenicnlum  vulgare,)  arrived  at  maturity,  showing 
the  carpophore  and  the  two  suspended  mericarps  or  hemicarps. 

bunches  from  the  branches  of  the  ash  and  maple,  commonly 
known  as  keys.  The  fruit  of  the  maple  consists  of  two  united 
Samara. 

The  Pome,  (pomum,  an  apple.)  This  is  a  fleshy  indehis- 
cent  fruit  with  a  superior  calyx,  which  is  therefore  adherent  to 
the  ovary.  In  the  mature  pome,  the  epicarp  and  calyx  are 
blended  together  and  form  along  with  the  mesocarp  the  thick 
cellular  and  edible  part  of  the  fruit,  whilst  the  endocarp 
enveloping  the  seeds  in  its  interior  takes  the  consistency  of 
parchment,  and  usually  forms  five  cavities  in  the  centre  of  the 
fruit. 

The  Drupe. — This  is  a  thick,  fleshy  and  indehiscent  fruit, 
containing  an  unilocular  nut,  as  in  the  plum  and  cherry. 
This  nut  is  formed  by  the  ossification  of  that  portion  of  the 
pericarp  which  is  called  the  endocarp,  which  in  this  case  forms 
a  strong  stony  envelope  around  the  seed.  In  drupaceous 
fruits,  such  as  the  peach  and  cherry,  the  epicarp,  mesocarp, 
and  endocarp  are  easily  distinguished  and  separated,  but  in 
the  nut  these  parts  are  all  so  much  ossified  and  blended 
together  as  to  be  indistinguishable.  The  nut  only  differs  from 
the  drupe  in  being  a  less  succulent  and  more  coriaceous 


THE   FRUIT.  205 

pericarp.     The  fruit  of  the  raspberry  and  blackberry  is  an 
aggregate  of  little  drupes  borne  on  a  common  receptacle. 

The  JBacca,  or  berry.  This  is  a  fleshy,  compound  fruit, 
which  is  pulpy  throughout.  This  name  usually  distinguishes 
such  fruits  as  the  gooseberry  and  currant,  in  which  the  calyx 
is  adherent  to  the  ovary  and  the  parietal  placentas.  The  seeds 
are  at  first  attached  to  the  placentas,  but  as  the  fruit  ripens 
they  become  detached  from  the  placentas,  which  finally  form 
that  pulp  which  fills  the  interior  of  the  berry  and  in  which  the 
seeds  are  imbedded.  The  term  berry  is  in  general  -  applied  to 
all  pulpy  fruits. 
The  principal  varieties  of  the  dehiscent  pericarp  are — 

1.  The  follicle  (folliculus,  a  little  bag.)     This  is  an  unilo- 
cular  fruit,  opening  longitudinally  by  a  single  suture,  the  ven- 
tral, into  one  valve,  which  represents  an  open  carpellary  leaf. 
The  seeds  are  attached  to  a  simple  sutural,  or  bi-partite  pla- 
centa, and  sometimes  become  free  at  the  moment  the  valves 
separate.     Follicles  are  very  seldom  solitary  fruits.     They  are 
usually  aggregated  on  a  short  receptacle,  and  form  a  verticil,  as 
in  the  Columbine, 

2.  The  legume  or  pod  (Legumen,  pulse),  is  a   dry   fruit, 
bi-valve,  opening  at  the  same  time  by  the  ventral  and  dorsal 
suture,  and  bearing  its  seeds  on  the  former.     In  the  bladder 
senna  (Colutea  arborescens),  the  legume  is  inflated,  and  retains 
its  leaf-like  character.     Fig.  90  is  a  lomentaceous  variety  of 
the  legume  to  which  reference  has  been  already  made,  and 
which  breaks  up  at  the  constrictions.     This  fruit  belongs  to 
all  the  family  of  the  Leguminosae  of  which  it  forms  the  prin- 
cipal character.     Examples — the  pea,  bean,  and  the  acacia. 

3.  The  capsule  (capsula,  a  little  chest.)     This  is  a  general 
name  for  all  dry  and  dehiscent  fruits  which  open  by  valves  or 

18* 


206          COMPOUND  ORGANS  OF  PLANTS. 

Fig.  101. 


Fig.  101.  The  Siliqua  of  the  Wall  Flower  (Cheiranthus  cheiri)  opening  by  two  ralrcs 
from  the  base  upwards.  The  two  placentas  bearing  the  seeds  on  their  surface,  remain 
in  tb«  middle  of  the  fruit,  with  a  replum  between  them. 

pores.  It  is  easy  to  imagine  from  this,  that  the  forms  of  the 
capsule  will  be  exceedingly  variable.  The  porous  capsule  is 
seen  in  the  poppy,  which  is  a  seed  vessel  of  a  woody  texture, 
proceeding  from  a  compound  ovary,  and  dehiscing  by  chinks 
which  may  be  seen  in  the  dry  fruit,  "just  beneath  the  over- 
hanging surface  of  its  numerous  radiating  stigmas.  Two  other 
varieties  of  the  capsule  are  worthy  of  a  particular  notice. 

4.  The  Siliqua  (siliqua  a  husk  or  pod.)  This  is  a  pod- 
shaped  capsule,  the  peculiar  fruit  of  Cruciferous  plants,  com- 
posed of  two  carpels  which  open  as  valves  from  below,  upwards. 
The  parietal  placenta,  before  the  period  of  dehiscence,  having 
been  united  together  by  a  plate  of  cellular  matter  termed  the 
replum,  which  forms  a  false  septum  across  the  cavity  of  the 
ruit,  separate  from  the  valves,  when  these  open  and  remain 


THE   FRUIT. 


207 


attached  to  the  replum,  in  the  axis  of  the  fruit.  These  pla- 
centa thus  united  together  by  the  replum,  frequently  remain 
after  the  fall  of  the  valves,  until  the  foliage  of  the  plant  finally 
decays. 


102. 


Fig.  102.  The  fruiting  branch  of  the  Shepherd's  purse  (Capsella  bursa  pastoris,) 
supporting  siliculse.  a.  Magnified  silicula,  opening  by  two  valves  from  the  back 
upwards,  each  valve  leaving  its  placenta  covered  with  seeds,  and  attached  to  the 
replum  in  the  centre  of  the  fruit. 

5.  The  Silicula.  This  is  simply  a  short  and  broad  Siliqua 
containing  sometimes  only  one  or  two  seeds.  It  is  also  peculiar 
to  Cruciferous  plants. 


208  COMPOUND   ORGANS   OF   PLANTS. 


CHAPTER    XVI. 

THE   STRUCTURE   OF   THE    SEED. 

THE  seed  of  Phanerogamous  plants  is  the  fecundated  ovule 
ripe  and  ready  for  germination,  enclosing  in  its  interior  a  plant 
in  miniature",  called  the  plantule  or  embryo,  which,  when  there 
are  the  suitable  conditions,  is  capable  of  reproducing  the  mother 
plant,  and  of  again  passing  through  precisely  the  same  phases 
of  development. 

The  seed  like  the  ovule  is  composed  of  a  kernel  or  nucleus, " 
usually  covered  by  two  cellular  integuments,  and   included 
under  the  general  name  of  episperm. 

The  episperm  or  proper  tegument  of  the  seed  is  the  coat 
•which  covers  it  exteriorly.  This  coat  is  formed  by  the  two 
membranes  which  we  have  seen  to  exist  in  the  ovule  at  the 
moment  of  fecundation,  viz.,  the  primine  and  secundine.  In 
a  great  number  of  cases  these  two  membranes  are  so  soldered 
together  that  the  episperm  is  thin  and  constitutes  only  a  simple 
membrane.  But  it  sometimes  happens  that  the  two  superposed 
membranes  of  the  episperm  are  distinct  enough;  and  when 
this  is  the  case  the  exterior  membrane  is  ordinarily  more  thick 
and  tough  than  the  interior  one,  immediately  enveloping  the 
seed.  To  distinguish  them  from  each  other,  the  former  is 
called  the  testa,  and  the  latter  the  tegmen.  These  two  mem- 
branes are  perfectly  distinct  in  the  episperm  or  seed  coat  of  the 
Castor  oil  plant  (Ricinus.) 

The  episperm  has  usually,  on  its  exterior  surface,  certain 
markings  which  correspond  to  those  mentioned  in  the  ovule. 


STRUCTURE   OF   THE    SEED.  209 

On  one  part  of  the  surface  of  the  episperm  we  see  constantly 
the  hilum,  a  scar  marking  the  point  by  which  the  seed  was 
attached  to  the  funiculus  or  placenta  whilst  in  the  pericarp 
(Fig.  103.)  a.  The  hilum  is  more  or  less  conspicuous  on  the  epis- 

Fig.  103. 


Fig.  103.  Leguminous  seed.  a.  The  hilum  under  the  form  of  a  linear  cicatrice.  6. 
The  micropyle. 

perm  of  all  seeds,  its  color  being  very  frequently  quite  different 
from  the  color  of  the  rest  of  the  surface  of  the  seed.  The 
hilum  is  very  conspicuous  in  the  bean  and  pea,  being  quite 
black  in  the  former.  It  is  by  the  hilum  that  the  nourishing 
vessels  of  the  pericarp  penetrate  the  seed.  They  traverse  the 
double  or  single  membrane  of  the  episperm,  and  enter  the 
oaucleus  or  kernel  by  the  chalaza,  a  term  applied  to  the  fibro- 
vascular  bottom  of  the  nucleus  or  kernel  where  it  unites  with 
the  episperm. 

On  the  surface  of  the  episperm,  we  perceive  frequently  very 
near  to  the  hilum  or  in  a  point  diametrically  opposite  to  it  a 
punctiform  opening  extremely  small  which  is  called  the  micro- 
pyle. (Fig.  103.)  b.  The  micropyle  is  simply  the  foramen  or  open- 
ing of  the  two  membranes  of  the  ovule  which  is  contracted  to  a 
point,  so  as  to  become  sometimes  hardly  perceptible.  The 
micropyle  may  be  readily  detected  in  the  pea  or  bean  in  the 
form  of  a  small  hole  or  point  which  in  this  instance  is  near  the 
hilum.  The  micropyle  always  corresponds  to  that  point  of  the 
nucleus  where  the  embryo  sac  is  formed,  and  the  summit  of 
which  gives  birth  to  the  embryonic  vesicle.  It  follows  from 


210         COMPOUND  ORGANS  OF  PLANTS. 

this  that  the  radicle  of  the  embryo  always  points  to  the  micro- 
pyle.  This  fact  the  student  may  readily  ascertain  by  dissect- 
ing any  seed  which  has  a  visible  micropyle  on  the  episperm, 
and  ascertaining  the  direction  of  the  radicular  extremity  of  the 
embryo.  He  will  find  it  invariably  pointing  to  this  very  spot. 

Sometimes  the  micropyle  entirely  disappears  from  the  sur- 
face of  the  episperm.  Its  place,  however,  may  be  readily 
ascertained.  If  the  skin  of  a  seed  be  carefully  examined  it 
will  be  usually  found  to  be  marked  with  lines  or  bands  which 
run  upwards  from  the  hilum.  These  lines  always  converge 
and  meet  in  the  micropyle,  so  that  by  following  them  with 
the  eye,  the  micropyle  may  be  frequently  discovered  on  the  epi- 
sperm, when  owing  to  its  minuteness  it  would  otherwise  escape 
detection. 

The  chalaza  is  more  or  less  visible  in  'all  anatropous  seeds, 
being  often  colored  and  of  a  denser  texture  than  the  surround- 
ing tissue.  At  the  apex  of  the  seed  of  the  orange  and  many 
other  plants,  it  may  be  perceived  on  the  episperm  in  the  form 
of  a  large  brown  spot.  In  orthotropous  and  campulitropous 
seeds  the  chalaza  is  directly  superposed  on  the  hilum,  with 
which  it  is  immediately  confluent,  but  in  all  anatropous  seeds 
it  is  placed  apart  from  the  hilum,  and  is  connected  with  a  vas- 
cular bundle  called  the  raphe,  which  forms  a  longitudinal  pro- 
minence more  or  less  conspicuous  on  the  episperm.  In  most 
plants  the  raphe  consists  of  a  single  line,  as  in  the  castor-oil 
plant,  but  in  the  orange  and  lemon  it  ramifies  upon  the  surface 
of  the  episperm.  Fig.  104. 

It  is  proper  to  remark  here,  that  the  terms  orthotropous, 
campulitropous,  and  anatropous,  employed  to  designate  the 
different  kinds  of  ovules,  are  equally  applicable  to  seeds,  all 
seeds  occurring  under  one  or  other  of  these  three  leading 
forms. 


STRUCTURE   OP   THE   SEED.  211 

Fig.  104. 


Fig.  104.  Anatropal  seed  of  the  Orange,  (Citrus  aurantium,)  opened  to  show  tbe 
cbalaza,  c,  which  forms  a  brown  spot  at  one  end ;  r,  raphe,  or  internal  funiculus  rami- 
fying in  the  rugose  or  wrinkled  testa  of  the  orange. 

On  the  outside  of  the  episperm  there  is  sometimes  an  addi- 
tional envelope  formed,  after  the  fertilization  of  the  ovule,  by 
an  expansion  of  the  funiculus  at  the  hilum.  This  funicular 
expansion,  which  covers  more  or  less  of  the  surface  of  the 
episperm,  is  termed  an  aril.  The  aril  is  very  conspicuous  in 
the  Spindle  tree  or  Burning  bush,  (Euonymus,)  where  it  forms 
a  beautiful  scarlet  envelope  to  the  seed.  The  tough,  fleshy 
and  lacerated  body  which  invests  the  seed  of  the  nutmeg, 
known  in  commerce  under  the  name  of  mace,  is  an  aril. 

The  nucleus,  or  kernel.  This  is  all  that  part  of  the  ripe 
seed  which  is  enveloped  by  the  episperm.  It  is  formed  by  the 
development  of  the  nucleus  of  the  ovule,  and  like  that  organ  is 
attached  to  the  episperm  by  its  base,  which  forms  the  chalaza. 
Generally,  in  the  ripe  seed  this  communication  is  destroyed. 
The  kernel  in  a  fecundated  seed  always  contains  an  embryo. 

In  exalbuminous  seeds,  after  fecundation,  the  embryo  takes  a 
considerable  development,  absorbing  into  its  cotyledons  the 
nutritive  matter  of  the  nucleus,  so  as  ultimately  to  constitute 
the  entire  kernel,  as  in  the  pea  and  bean.  In  albuminous 
seeds,  the  embryo  appears  to  be  arrested  in  its  growth  whilst 
yet  in  a  minute  and  rudimentary  condition,  developing  only  so 


212  COMPOUND   ORGANS   OF   PLANTS. 

far  as  just  to'  exhibit  its  component  organs,  and  remaining 
imbedded  in  the  nutritive  matter  of  the  nucleus  which  is  unab- 
sorbed.  The  embryo  of  the  Marvel  of  Peru,  (Mirabilis,)  of 
the  maize,  buckwheat,  and  the  whole  of  the  cerealia  con- 
tinues in  this  rudimentary  condition. 

The  albumen,  termed  by  some  authors  the  perisperm,  and 
also  the  endosperm,  when  present  in  the  kernel  varies  in  its 
consistence  according  to  the  nature  of  the  deposit  and  the  state 
of  the  cells.  It  consists  of  a  mass  of  cells  without  any  appear- 
ance of  vessels,  which  may  be  thin  and  dry  and  contain  a  great 
quantity  of  fecula  or  starch,  as  in  the  corn  and  the  other 
grasses;  or  thick  and  fleshy,  containing  juices  of  various  kinds, 
as  in  the  cocoa-nut  and  EuphorbiaceaB ;  or  finally,  the  cells 
may  be  of  a  horny  or  ligneous  nature,  as  in  the  coffee  and  vege- 
table ivory,  (Phytelephas.)  The  quantity  of  albumen  in  seeds 
depends  on  the  extent  to  which  embryonic  development  is  car- 
ried. When  the  embryo  is  small  the  albumen  is  abundant,  as  in 
the  seed  of  the  monkshood,  (Aconitum,)  Fig,  105,  where  e  repre- 

Fig.  105.  Fig.  106. 


Fig.  106.    Vertical  section  of  the  achenium  of  the  nettle,  (Urtica,)  showing  the  em- 
bryo nearly  filling  the  achenium,  r  radicle ;  pi  plumule ;  t  testa,  or  integument. 

sents  the  embryo.     When  the  embryo  is  large,  as  in  the  nettle, 
Fig.  106,  the  albumen  is  very  scarce.     In  the  Labiatae,  the 


STRUCTURE   OF   THE    SEED.  213 

albumen  is  reduced  to  a  mere  pellicle  by  the  great  development 
of  the  embryo. 

The  embryo  is  the  most  important  part  of  the  seed,  and  the 
final  product  of  the  vegetative  functions.  When  this  is 
formed,  and  the  seed  is  fully  ripe,  a  pause  in  growth  takes 
place,  and  the  embryo  which  is  the  future  plant  in  the  first 
stage  of  its  development,  will  sometimes  remain  for  a  long  time 
in  an  apparently  dead  condition  enveloped  in  the  folds  of  the 
seed,  until  suitable  circumstances  arouse  its  dormant  vitality. 
The  embryo  is  a  complete  plant  in  miniature,  and  therefore 
offers  the  same  general  disposition  of  its  parts  as  that  which 
we  have  already  noticed  in  the  adult  plant.  Thus  we  distin- 
Fig.  107. 


Fig.  107.  Embryo  of  the  Pea  (Pisum,)  laid  open  to  show  its  different  parts.  This 
embryo  occupies  the  whole  interior  of  the  seed,  c,  c,  its  two  fleshy  cotyledons ;  p,  the 
plumule ;  r,  the  radicle ;  g,  the  gemmule ;  /,  the  depression  left  by  the  gemmule  in 
the  cotyledon.  This  embryo  is  dicotyledonous  and  hypogeal,  the  cotyledons  remaining 
below,  during  germination. 

guish,  in  every  young  plantule,  an  axophyte  more  or  less 
developed,  with  the  usual  appendages,  root,  stem  and  leaves,  all 
in  a  rudimentary  state,  and  all  manifesting  an  identity  in  their 
incipient  vital  action  with  the  same  phenomena  in  the  adult 
plant.  The  little  embryo  axophyte  commences  to  develope  at 
its  two  extremities  in  two  opposite  directions,  and  puts  forth 
laterally  its  rudimentary  leaves ;  that  portion  which  ascends 
is  called  the  plumule,  that  which  descends  the  radicle ;  the 

19 


214 


COMPOUND  ORGANS  OF  PLANTS. 


rudimentary  leaves  are  named  cotyledons,  and  the  little  bud 
by  which  the  plumule  is  terminated,  is  called  the  gemmule, 
(Fig.  107.) 

Before  we  examine  in  succession  these  four  parts  of  the 
embryo,  let  us  consider  their  relative  positions  with  respect  to 
the  other  parts  of  the  seed. 

When  albumen  is  present  in  the  seed  along  with  the  embryo, 
the  embryo  may  either  lie  in  its  midst  directly  in  the  axis  of 
the  seed  as  in  the  pansy,  Viola  tricolor,  (Fig.  108,)  when  it  is 
axial ;  or  it  may  surround  the  albumen  itself,  instead  of  being 
surrounded  by  it  as  in  the  Marvel  of  Peru,  (Fig.  109,)  when 
it  is  peripherical.  In  the  grasses,  maize,  wheat  and  all  the 
cerealia,  the  embryo  lies  external  to  the  albumen  on  one  of 
the  sides  of  the  seed,  having  been  apparently  forced  into  this 
position  by  the  irregular  development  of  the  parts  of  the  seed, 
when  it  is  abaxial.  (Fig.  110,  Indian  corn.) 

Fig.  108.  Fig.  109.  Fig.110. 


Fig.  108.  Vertical  section  of  the  seed  of  the  Pansy.  The  seed  is  anatropal ;  the  em- 
bryo homotrope.  ch,  chalaza  to  which  co,  the  cotyledons  point ;  pi,  the  plumule ;  h, 
the  hilum;  al,  the  albumen  surrounding  the  embryo  which  it  will  be  perceived  is 
axial ;  r,  the  raphe  connecting  the  hilum  or  base  of  the  seed  with  the  chalaza  or  base 
of  the  nucleus. 

Fig.  110.  Vertical  section  of  a  grain  of  Indian  corn  (Zea  Mays,)  r,  the  radicle ;  p,  the 
plumule ;  c,  the  cotyledons. 

The   embryo   is   sometimes   straight    but    very  frequently 
curved  in  a  variety  of  ways,  its  curvature  depending  on  that 


STRUCTURE  OP    THE    SEED.  215 

of  the  seed.  In  orthotropous  and  anatropous  seeds  the  embryo 
is  usually  straight ;  in  such  seeds  as  are  campylotropous  it  is 
curved.  Whatever  may  be  the  form  of  the  seed,  the  radicle 
always  points  to  the  micropyle  and  the  cotyledons  to  the 
chalaza  or  some  point  in  its  vicinity.  This  important  law 
being  remembered,  it  is  only  necessary  to  ascertain  the  situa- 
tion of  the  micropyle  with  respect  to  the  chalaza  on  the 
surface  of  the  episperm,  and  the  character  not  only  of  the 
seed,  but  the  exact  position  of  the  embryo  within  its  folds  is 
at  once  determined  without  any  further  trouble.  Thus  in  the 
orthotropous  seed  of  the  nettle,  (Fig.  106,)  we  know  that  the 
micropyle  is  directly  opposite  to  the  hilum  and  chalaza,  which 
is  the  base  of  the  seed,  the  radicle  therefore  points  to  the  apex 
of  the  seed,  and  its  plumule  to  the  base,  and  the  embryo  is 
antitrope  (avti,,  opposite,  T-psytw,  I  turn,)  or  inverted.  But  in 
anatropal  seeds,  as  in  the  pansies,  (Yiola  tricolor,  Fig.  108,)  we 
see  on  the  surface  of  the  episperm  the  micropyle  close  to  the 
hilum  or  base  of  the  seed  and  the  chalaza  at  its  apex  or  opposite 
extremity ;  the  radicle  or  base  of  the  embryo  therefore  points 
to  the  base  of  the  seed  and  its  cotyledons  to  the  apex,  and  the 
embryo  lies  in  the  seed  in  its  natural  position ;  that  is  to  say, 
it  is  erect  or  homotrope,  (6/totoj,  like,  and  -^'rfco,  I  turn.)  In 
the  campylotropal  or  curved  seed,  the  base  is  not  displaced,  the 
seed  curves  on  itself,  and  the  micropyle  approaches  the  hilum 
and  chalaza,  which  is  still  confluent  with  it;  from  this  we 
know  that  the  cotyledonary  and  radicular  extremities  of  the 
embryo  also  approach  each  other,  or  the  embryo  is  amphi- 
trope  (ap$l,  around,  and  tpertu,  I  turn),  or  follows  the  curva- 
ture of  the  seed,  (Fig.  109.) 

Let  us  now  examine  in  particular  each  of  the  parts  which 
constitute  the  embryo. 


216          COMPOUND  ORGANS  OP  PLANTS. 

Fig.  111. 


Fig.  111.  Vertical  section  of  the  campulitropous  seed  of  the  red  campion  (Lychnis,) 
showing  the  curved  embryo. 

1.  The  radicle. — This  constitutes  the  lower  extremity  of  the 
embryo,  which  in  developing  forms  the  root,  or  which  gives  birth 
to  it.  It  appears  very  often  under  the  form  of  a  little  round  or 
conical  teat.  This,  by  germination,  sometimes  elongates  and 
becomes  the  body  of  the  root.  Its  extremity  continues  naked 
and  afterwards  divides.  This  mode  of  development,  which  is 
characteristic  of  Phanerogamous  plants  having  two  seed-lobes  or 
cotyledons,  is  termed  exorhizal,  (t'|«  outwards,  and  /'{£a  a  root.) 
At  other  times  the  radicle  in  germinating,  after  having  taken 
a  certain  degree  of  elongation,  stops  the  teat  at  its  extremity, 
becomes  covered  with  a  cellular  layer  as  with  a  sheath,  through 
which  breaks  forth  one  or  more  fibres,  which  constitute  the 
true  roots  of  the  embryo.  This  kind  of  development,  which 
is  peculiar  to  such  phanerogamous  plants  as  have  only  one  seed- 
lobe,  is  termed  endorhizal,  (svdov  within,)  and  the  sheath  formed 
at  the  extremity  of  the  radicle  teat  is  called  the  coleorhiza, 
(xota'oj  a  sheath,  and  Xa  a  root-  Fig.  112>  shows  both  kinds 
of  germination. 

The  plumule  forms  with  the  radicle  the  axis  of  the  embryo. 
It  is  developed  after  the  radicle,  which  it  surmounts,  and  with 
which  it  is  united.  It  exists  only  in  Dicotyledonous  embryos, 
and  is  terminated  at  its  summit  by  the  gemmule.  It  is  the 
plumule  which,  by  its  development,  produces  the  stem.  It 
commences  from  the  point  where  the  cotyledons  are  attached 


STRUCTURE    OF   THE    SEED. 
Fig.  112.  Fig.  113. 


217 


Fig.  112.  a  shows  theexorhizal  germination  of  the  Dicotyledonous  seed  of  the  orange; 
c,  the  cotyledon;  g,  the  first  pair  of  aerial  leaves;  r,  the  radicle  naked  and  without  a 
eheath. 

Fig.  113.  Seed  of  oats  sprouting.'  r,  roots  passing  through  the  sheath,  sh,  from  the 
single  cotyledon  c.  g,  The  young  leaves  and  stalk. 

to  it,  and  which  it  raises  with  it  above  the  earth,  when  its 
elongation  operates  from  its  base. 

The  cotyledons. — These  are  the  lateral  appendages  of  the 
embryo  axophyte.  The  cryptogamia  have  no  cotyledons  in  their 
embryos,  which  are  therefore  acotyledonous.  The  embryo  in 
such  cases  is  called  a  spore,  and  as  it  gives  off  roots  indiffer- 
ently from  any  part  of  its  surface,  and  from  a  fixed  point,  it 
is  termed  heterorizal,  (stspos  diverse.)  Plants  possessing  coty- 
ledons in  their  embryo  are  termed  cotyledonous.  If  we  examine 

19* 


21S          COMPOUND  ORGANS  OF  PLANTS. 

the  embryo  of  the  bean,  the  pea,  or  the  oak,  we  shall  see  a  coty- 
ledonary  body  formed  of  two  cotyledons.  The  embryo  which 
presents  such  a  conformation  is  a  Dicotyledonous  embryo.  If, 
on  the  contrary,  we  examine  the  embryo  of  the  wheat  or  the 
maize,  of  the  iris  or  the  palm,  we  shall  find  a  simple  coty- 
ledonary  body,  formed  by  a  single  cotyledon.  The  embryo 
in  this  instance  is  termed  a  monocotyledonous  embryo. 
The  character  drawn  from  the  number  of  cotyledons  is  of  the 
highest  importance,  because  it  divides  all  phenogamous  plants, 
or  those  which  are  provided  with  flowers  properly  speaking, 
into  two  grand  branches,  the  Monocotyledons  and  the  Dicoty 
ledons,  which  differ  not  only  in  the  structure  of  their  embryo, 
but  in  the  special  organization  of  all  their  other  parts. 

A  certain  number  of  Phanerogamous  plants  are,  however, 
apparently  exceptions  in  the  structure  of  their  embryo  to  these 
two  grand  divisions.  The  cone-bearing  plants,  for  example, 
such  as  the  spruce,  fir,  and  larch,  have  not  one  or  two,  but 
sometimes  six,  nine,  twelve  and  even  fifteen  verticillate  cotyle- 
dons, which  resemble  in  their  linear  form  and  verticillate 
arrangement,  the  clustered  and  fascicled  leaves  of  the  larch. 
To  such  embryos  the  term  polycotyledonous  has  been  applied, 
but  M.  Duchatre  has  proved  that  these  polycotyledonous 
embryos  are  only  Dicotyledonous  embryos,  whose  two  cotyle- 
dons are  deeply  divided  into  a  number  of  segments.  Therefore, 
it  is  proper  to  retain  them  among  the  Dicotyledonous  embryos 
of  which  they  are  only  a  variety. 

In  exalbuminous  embryos,  that  is  to  say,  those  which  are 
immediately  covered  by  the  episperm  or  seed  coat,  the  cotyle- 
dons are  excessively  thick  and  fleshy,  and  their  albuminous 
contents  furnish  to  the  young  and  germinating  embryo  the 
first  materials  of  its  nutrition.  In  such  seeds  as  are  albumi- 


STRUCTURE    OF    THE    SEED.  219 

nous,  on  the  contrary,  the  cotyledons  are  thin  and  membrana- 
ceous,  retaining  in  a  great  measure  the  appearance  of  leaves, 
in  the  midst  of  the  surrounding  albumen. 

At  the  period  of  germination  the  cotyledons  separate  from 
the  integuments  of  the  seed,  and  either  appear  above  the 
ground,  different  in  form  from  the  other  leaves  of  the  plant,  or 
they  remain  hidden  in  the  earth  without  showing  themselves,  as 
in  the  pea  and  the  horse  chesnut,  until  they  finally  decay.  In 
the  former  case,  they  are  epigeal  (liti  upon  or  above,  and  ys'a 
the  earth;)  in  the  latter  case  they  are  hypogeal  (vrto  under.) 

The  gemmule,  is  the  little  bud  at  the  summit  of  the  plumule. 
Like  all  other  buds,  it  is  composed  of  a  little  axis  continuous 
with  that  of  the  embryo,  and  certain  minute  rudimentary 
leaves  which  represent  the  first  leaves  which  the  embryo  is 
going  to  develope.  In  general,  in  Dicotyledons  the  gemmule 
is  placed  between  the  two  cotyledons,  which  in  being  applied 
one  against  the  other,  cover  and  hide  it  completely.  It  is 
therefore  necessary  to  separate  the  cotyledons  in  order  to  see 
the  gemmule.  In  Monocotyledons  embryos,  the  plumule  is 
absent  and  the  gemmule  is  placed  within  the  sheathing  base  of 
the  cotyledonary  leaf,  and  situated  as  it  were,  on  one  of  its 
sides.  In  developing,  the  gemmule  gives  birth  to  the  aerial 
portion  of  the  stem,  and  its  unfolding  rudimentary  leaves  soon 
take  in  succession  the  form,  position  and  size  of  those  leaves 
which  are  peculiar  to  the  adult  plant. 


220  COMPOUND  ORGANS  OF  PLANTS. 

; 

CHAPTER    XVII. 

ON  THE  DISPERSION  AND  GERMINATION  OF  SEEDS. 

IT  must  be  obvious  that  the  immense  quantity  of  seed  which 
plants  generally  produce,  could  never  germinate  in  their  imme- 
diate neighborhood,  and  therefore,  as  the  seed  ripens,  the  peri- 
carp gradually  assumes  such  an  organization  as  is  calculated  to 
effect  its  dispersion  or  removal  to  a  more  distant  locality.  The 
dissemination  of  the  seeds  is  the  result  of  the  peculiar  organi- 
zation of  their  pericarp  or  seed-vessels,  rather  than  of  the  seeds 
themselves,  which  in  this  respect  present  some  of  the  most 
interesting  and  beautiful  contrivances  in  nature. 

Sometimes  the  pericarp  opens  elastically  with  a  spring-like 
mechanism,  and  discharges  the  seed  contained  in  its  cavity  to 
a  considerable  distance.  The  seeds  of  the  castor  oil  plant,  of 
the  common  garden  balsam,  and  of  the  common  furz,  or  whin- 
bush  of  Europe,  are  separated  from  their  pericarps  in  this 
manner.  In  Hura  crepitans,  a  plant  which  grows  in  the  West 
Indies  and  in  South  America,  the  seeds  are  projected  from 
the  strong  bony  envelope  of  the  pericarp  as  soon  as  it  opens, 
which  it  does  with  immense  force  and  with  a  report  as  loud  as 
a  pistol. 

The  pericarps  of  the  thistle,  dandelion,  and  other  species  of 
Composite,  have  attached  to  them  a  beautiful  stellate  down; 
contrivances  which  are  evidently  intended  to  catch  the  wind, 
and  by  means  of  which  they  are  removed  when  fully  ripe  from 
off  the  surface  of  the  receptacle  of  these  plants,  and  wafted  to 
a  distance  to  spots  favorable  to  their  germination.  The  peri- 
carps to  which  these  appendages  are  attached,  will  sometimes 


DISPERSION  AND   GERMINATION    OF   SEEDS.  221 

travel  for  miles  until  a  shower  of  rain  or  a  humid  atmosphere 
causes  the  tuft  to  collapse,  when  the  pericarp  falls  to  the 
ground.  In  some  instances,  as  in  the  thistle,  this  down  pro- 
jects directly  from  the  surface  of  the  pericarp  like  the  feathers 
of  a  shuttle-cock;  in  the  dandelion  and  goatsbeard  it  is  sup- 
ported upon  a  stalk  which  elevates  it  above  the  seed.  In  the 
last  plant  each  fine  hair  of  the  tuft  is  itself  a  feather,  forming 
altogether  one  of  the  most  elegant  and  perfect  objects. 

In  other  species  the  pericarps  are  furnished  with  hooked 
hairs,  which  cover  their  entire  surface,  as  in  galium  and  bur- 
dock, by  means  of  which  they  cling  to  the  bodies  of  men  and 
animals,  and  are  thus  scattered  far  and  wide.  In  autumn  it  is 
impossible  to  traverse  the  woods  0£  marshes  without  having 
such  pericarps  forced  upon  our  attention.  The  achenia  of 
Bidens  bipinnata,  or  the  Spanish  needles,  are  especially  trou- 
blesome. The  achenia  of  this  plant  are  surmounted  with  three 
or  four  persistent  awns,  which  are  downwardly  barbed,  and  by 
means  of  which  they  very  readily  adhere  to  the  dress  of  the 
traveller.  How  little  are  persons  aware  when  they  brush  off 
these  troublesome  intruders,  in  some  distant  locality  to  which 
they  have  unwillingly  carried  them,  that  they  are  fulfilling  the 
grand  and  secret  purposes  of  nature  ! 

Occasionally,  as  in  the  Asclepias  or  milkweed,  and  the  Epi- 
lobium  or  willow-herb,  the  seeds  themselves  are  furnished 
with  the  coma  or  tufts  of  hairs,  by  means  of  which,  on  the 
dehiscence  of  the  pericarp,  they  are  lifted  by  the  wind  out  of 
its  cavity  and  carried  away  sometimes  to  a  great  distance  from 
the  parent  plant. 

Birds,  too,  are  important  agents  in  the  diffusion  of  seeds. 
It  is  well  known  that  the  seeds  of  numerous  berries  and  small 
fruits  will  grow,  though  they  have  passed  through  the  bodies 


222  COMPOUND  ORGANS  OF  PLANTS. 

of  birds.  It  is  in  this  way  that  Phytolacca  decandra,  or  the 
common  pokeweed,  appears  to  have  been  dispersed  over  the 
whole  of  North  America.  The  berries  of  this  plant  are  eaten 
by  the  robin,  the  thrush,  the  wild  pigeon,  and  many  other 
birds,  which  thus  carry  them  hundreds  of  miles  from  the  plant 
which  produced  them.  In  this  manner  we  can  account  for  a 
fact  which  every  practical  botanist  and  observer  of  nature  must 
have  noticed,  viz. :  the  sudden  appearance  of  a  single  plant  in 
a  place  where  its  species  was  entirely  unknown  before. 

Some  pericarps  are  conveyed  by  the  rivers  into  which  they 
fall,  or  by  the  waves  of  the  ocean,  many  hundreds  or  thousands 
of  miles  from  the  countries  which  originally  produce  them.  In 
this  manner  many  of  the  native  plants  of  France,  Spain,  and 
other  adjacent  countries,  have  been  naturalized  in  England ; 
and  the  pericarps  of  tropical  climates  are  conveyed  to  the  coasts 
of  Norway  and  Scotland.  The  foreign  pericarps  which  are 
annually  left  on  the  Norway  coast,  are  principally  cashew- 
nuts,  bottle-gourds,  cocoa-nuts,  and  the  fruit  of  the  dogwood 
tree.  These  are  often  in  so  recent  a  state,  that  they  would 
unquestionably  vegetate  were  the  climate  favorable  to  their 
growth  and  existence.  When  carried  to  countries  better  suited 
to  their  nature,  they  germinate  and  colonize  with  a  new  race  of 
vegetables  the  land  on  which  the  ocean  has  cast  them.  In  this 
manner  it  is  that  the  coral  islands,  as  soon  as  they  appear  above 
the  waves  of  the  Pacific,  are  speedily  covered  with  a  crop  of 
luxuriant  vegetation.  The  cocoa-nut  is  well  adapted  for  this 
purpose,,  as  it  grows  luxuriantly  in  salt  water,  and  it  is  proba- 
bly the  first  arborescent  species  which  vegetates  on  these  newly- 
formed  lands. 

Most  of  the  seeds  thus  carried  abroad  never  germinate  at 
all,  as  they  either  fall  into  situations  unfavorable  to  their 


DISPERSION   AND   GERMINATION   OP    SEEDS.'  223 

growth,  or  upon  a  soil  which  is  already  pre-occupied  by  other 
plants.  All  the  plants  of  a  given  district  may  fee  regarded  as 
at  war  with  each  other.  The  arborescent  species  prevent,  by 
the  extent  of  soil  which  they  occupy,  the  vegetation  of  species 
of  a  humbler  growth.  Each  has  "to  struggle  into  existence 
against  a  host  of  competitors,  for  nature,  although  she  has 
been  prolific  of  the  seeds  of  life,  has  limited  the  supply  of 
room  and  food.  A  number  of  ferns,  for  example,  which  may 
be  growing  on  a  hill-side,  will,  by  their  pre-occupation  of  the 
soil,  successfully  maintain  their  ground  against  all  other 
intruders  for  ages,  notwithstanding  the  facilities  afforded  to 
other  plants  for  the  dispersion  of  their  seeds.  If  any  chance 
seed  should  be  borne  to  this  spot  by  any  of  the  agencies  which 
we  have  enumerated,  or  by  other  causes,  it  cannot  germinate 
among  them,  as  they  absorb  all  the  food  from  the  soil. 

The  seeds  which  have  been  thus  unfavorably  located,  retain 
their  vitality  for  a  longer  or  shorter  period  of  time.  Such  as 
have  very  thin  and  delicate  integuments,  will  lose  their 
germinating  power  after  a  few  weeks'  exposure ;  so  also  oleagi- 
nous seeds  will  in  general,  decay  much  sooner  than  such  as 
contain  albumen.  Other  seeds,  on  the  contrary,  will  retain 
their  vitality  for  an  indefinite  period  of  time.  This  is  the  case 
with  the  Leguminous  plants,  the  seeds  of  which  may  be  kept 
for  years  without  any  material  detriment  to  their  germinating 
power.  Peas  taken  from  the  herbarium  of  Tournefort,  where 
they  had  remained  for  more  than  one  hundred  years,  were 
made  to  germinate  in  the  botanical  gardens  at  Paris.  Those 
changes  by  which  the  ovule  is  changed  into  the  mature  seed, 
appear  to  be  all  made  with  a  special  reference  to  any  mishaps 
which  may  befall  it  when  thrown  on  the  charity  and  care  of 
nature  by  the  parent  plant,  as  well  as  to  provide  it  with  a 


224  COMPOUND    ORGANS   OF   PLANTS. 

store  of  nutriment  on  which  it  may  subsist  during  the  early 
stages  of  its  development. 

When  the  plant  approaches  the  close  of  its  allotted  period  of 
life,  it  is  surprising  with  what  care  provision  has  been  made 
for  the  continuation  of  the'species,  as  if  nature  had  determined 
to  secure  it,  if  possible,  an  immortality  of  existence  upon  the 
earth's  surface.  Hence  not  only  the  beautiful  contrivances  to 
effect  the  removal  of  the  seed  to  spots  favorable  for  its  germi- 
nation, but  also  the  immense  quantity  of  seed  which  the  dying 
plant  produces.  On  a  specimen  of  the  castor  oil  plant,  which 
the  author  cultivated  in  his  garden,  he  counted  ten  clusters  of 
pericarps,  or  seed-vessels;  each  cluster  produced  upwards  of 
fifty  pericarps,  and  each  pericarp  contained  three  seeds.  The 
total  number  of  seeds  produced  by  the  plant  was,  therefore, 
10x50x3=1500.  Each  of  these  seeds,  be  it  remembered, 
contained  within  its  folds  an  incipient  repetition  of  the  parent 
plant  in  the  form  of  a  young  embryo.  Supposing  each  seed  to 
germinate,  and  the  plants  to  arrive  at  maturity,  the  product  of 
the  next  season  would  be  1500x1500=2,250,000  seeds  !  In 
other  plants,  the  first  crop  of  seeds  is  still  greater.  It  has 
been  calculated  that  the  sunflower  produces  4000  and  a  single 
thistle  24,000  seeds  the  first  year;  therefore  the  second 
year's  crop  would  amount  to  16,000,000  of  seeds  in  the  former, 
and  576,000,000  of  seeds  in  the  latter  instance.  How  immense 
the  amount  of  vegetable  life  which  may  spring  from  a  single 
seed !  Happily  for  mankind,  every  vegetable  embryo  is  not 
destined  to  give  rise  to  a  future  progeny.  Millions  of  seeds, 
or  vegetable  embryos,  are  called '  into  existence,  but  their 
incipient  life  is  speedily  destroyed  by  a  variety  of  causes. 
Were  it  not  for  the  operation  of  these  causes,  by  which  the 
species  is  kept  within  prescribed  limits,  such  is  the  fecundity 


DISPERSION   AND   GERMINATION   OF    SEEDS.  225 

of  nature,  that  there  can  be  no  doubt  that  the  seed  from  a 
single  thistle  or  dandelion  would,  in  the  course  of  a  few  years, 
be  sufficient  to  cover  with  plants  not  only  every  square  inch  of 
the  superficies  of  our  own  world,  but  the  entire  surface  of  every 
other  planet  in  the  solar  system. 

But  although  nature  has  been  thus  careful  to  ensure  a  repe- 
tition of  their  beautiful  and  evanescent  forms,  all  plants 
multiply  within  prescribed  limits,  which  they  cannot  pass. 
Fecundity  is  therefore  no  barrier  to  the  variety  which  every- 
where prevails,  which  is  the  principal  charm  of  the  vegetable 
creation,  and  from  which  we  derive  so  much  instruction  in  the 
study  of  their  individual  forms. 

When,  however,  the  seed  falls  into  a  soil  favorable  to  its 
germination,  it  will  grow  and  become  a  plant,  running  through 
all  the  phases  of  the  vegetation  of  its  predecessor. 

We  have  only  now  to  lay  before  the  student  the  conditions 
which  are  necessary  to  germination,  and  the  interesting  series 
of  phenomena  connected  with  the  evolution  of  the  young 
plantule  from  its  integuments. 

The  exterior  agents  indispensable  to  germination  are  water, 
air,  and  heat. 

Water  is  necessary  in  germination  as  in  all  the  other  pheno- 
mena of  vegetable  life.  It  penetrates  into  the  substance  of  the 
seed,  softens  its  envelopes,  and  makes  the  embryo  swell.  It 
therefore  places  the  seed  in  the  conditions  which  are  most 
favorable  for  its  development.  As  soon  as  germination  com- 
mences, it  dissolves  the  dextrine  and  the  other  soluble  prin- 
ciples which  exist  in  the  seed,  and  which  are  formed  by  the 
transformation  of  the  starch,  and  conveys  these  nutritive  mate- 
rials to  the  young  embryo. 

Air  as  well  as  water  is  also  necessary.  Experiments  show 
20 


226          COMPOUND  ORGANS  OP  PLANTS. 

that  seeds  will  not  germinate  in  vacuo,  or  in  a  space  from 
which  the  air  has  been  artificially  removed.  Hence  seeds 
buried  too  deeply  in  the  soil  will  not  germinate,  as  the  air 
cannot  get  access  to  them,  but  if  by  any  natural  or  artificial 
causes  they  are  brought  into  the  superficial  beds,  germination 
very  soon  commences.  It  is  thus  that  we  see  suddenly  appear 
in  a  locality  certain  plants  which  did  not  grow  there  before,  as 
for  instance,  when  a  waste  field  is  cultivated. 

It  is  the  oxygen  of  the  air  which  acts  principally  in  germina- 
tion ;  for  seeds  plunged  in  pure  nitrogen  or  hydrogen  neither 
germinate  nor  develope.     It  is  by  the  absorption  of  oxygen 
that  the  starch  which  remains  in  the  nucleus,  or  which  has  been 
absorbed  into  the  cotyledons,  is  rendered  soluble  and  nutritive. 
It  is  well  known  that  starch  is  quite  insoluble  in  cold  water. 
By  what  remarkable  operation?  in  vegetation  does  it  become 
soluble,  so  as  to  be  dissolved  and  transported  to  all  parts  of  the 
plant  ?     This  question  we  now  proceed  to  answer.     Starch  is 
profusely  spread  through  all  the  organs  of  the  plant,  and  is 
accumulated  especially  in  the  seed,  as  a  store  of  nutriment  on 
which  the  young  embryo  subsists,  until  such  times  as  its  roots 
and  leaves  are  sufficiently  developed  for  the  accumulation  of  its 
proper  food  from  the  earth  and  atmosphere.     But  in  order 
to  its  assimilation  by  the  young  embryo  it  is  necessary  that 
the  starch  in   the  cotyledons  or  nucleus  should   be  rendered 
soluble.    When  the  temperature  and  other  conditions  are  faVor- 
able,  a  vegetable  secretion  termed  diastase  forms  itself  in  all  the 
cells  which  contain  starch.     This  diastase  possesses  the  sin- 
gular property  of  transforming  starch  into  a  soluble   gum 
termed  dextrine,  which  the  water  is  able  to  carry  to  all  parts 
of  the  plant.     The  action  of  the  oxygen  of  the  air,  through 
the  secretion  of  the  diastase,  having  thus  changed  the  starch 


DISPERSION   AND  GERMINATION    OP    SEEDS.  227 

to  dextrine,  and  the  continuation  of  the  same  process  con- 
verting some  dextrine  into  sugar,  which  being  dissolved  by 
the  water,  also  penetrates  all  the  parts  of  the  embryo,  thus 
induces  the  necessary  nutritive  and  germinating  processes. 

The  starch  contained  within  the  folds  of  the  seed,  is  there- 
fore at  the  end  of  a  certain  time,  completely  re-absorbed. 
This  disappearance  of  the  starch  is  the  result  of  its  combus- 
tion by  the  embryo,  or  of  its  slow  conversion  principally  into 
carbonic  and  other  vegetable  acids,  and  in  part  into  cellulose. 

Heat  is  therefore  evolved  during  germination,  and  a  certain 
amount  of  it  becomes  indispensable  to  the  vital  action  of  the 
seed.  Placed  in  the  midst  of  a  temperature  below  zero,  it 
remains  benumbed  and  stationary  even  under  the  influence  of 
air  and  humidity.  But  a  mild  temperature  accelerates  the 
development  of  all  the  phenomena  of  vegetation.  Hence  it  is 
that  the  gardener  is  accustomed  to  hasten  the  development  of 
such  exotic  grains  as  it  is  his  interest  to  cultivate,  by  sowing 
them  in  a  hot  bed,  and  by  consequence  surrounding  them  with  a 
humid  and  artificial  heat.  But  it  is  necessary  that  this  tem- 
perature does  not  pass  certain  limits,  otherwise,  so  far  from 
hastening  the  development  of  the  seeds,  it  will  dry  up  and 
destroy  the  principle  of  life  within  them. 

The  degree  of  heat  required  to  excite  the  vitality  of  the 
embryo,  varies  from  50°  to  80°  (Fahrenheit,)  for  the  plants  of 
temperate  climates.  The  seeds  of  tropical  plants  require  a 
much  higher  heat  to  call  them  into  action,  varying  from  90°  to 
110°  (Fahrenheit,)  and  occasionally  a  more  elevated  tempera- 
ture than  even  this,  is  found  to  be  necessary. 

Light  exercises  an  unfavorable  influence  on  germination. 
This  must  necessarily  be  its  effect  on  the  germinating  seed,  for 
we  have  shown  that  the  absorption  of  the  carbonic  acid  of  the 


228          COMPOUND  ORGANS  OP  PLANTS. 

atmosphere,  the  assimilation  of  the  carbon  and  the  evolution 
of  the  oxygen,  are  processes  which  are  greatly  forwarded  by 
this  agent.  Now  all  these  processes  are  just  the  reverse  in 
germination,  for  oxygen  is  absorbed  and  carbonic  acid  is  elimi- 
nated. It  is  not  true  that  seeds  will  not  germinate  unless  pro- 
tected from  the  influence  of  light,  since  every  day  we  see  plants 
germinating  very  well  and  with  considerable  rapidity,  on  fine 
sponges,  on  sand,  or  other  bodies  from  which  they  can  imbibe 
water ;  but  it  is  nevertheless  true  that  a  strong  light  will  greatly 
retard  whilst  darkness  will  favor  the  germinating  process. 

The  general  phenomena  of  germination  may  be  thus  summed 
up.  When  there  are  the  suitable  conditions  of  temperature, 
air  and  moisture,  the  first  phenomena  which  we  observe  in  the 
germinating  seed  is  the  swelling  and  softening  of  the  envelopes 
which  covered  it.  These  become  distended  with  moisture,  and 
ultimately  ruptured  in  a  more  or  less  irregular  manner,  as  the 
swelling  of  the  seed  increases.  About  the  same  time  that  the 
seed  commences  to  be  distended  with  moisture,  it  attracts 
oxygen  from  the  atmosphere.  This  oxygen  induces  the  formation 
of  the  diastase,  which  acts  on  the  starch  contained  in  the  coty- 
ledons, converting  it  into  dextrine  and  sugar,  which  dissolved 
by  water,  are  conveyed  to  all  parts  of  the  young  embryo.  The 
bulk  of  this  sugar  is  converted  into  carbonic  acid,  whilst  the 
remainder  or  dextrine,  is  organized  into  cellulose.  •  Therefore, 
instead  of  taking  in  the  materials  of  nutrition  from  the  earth 
and  atmosphere,  or  assimilating  externally  in  germination  as  in 
the  process  of  flowering,  the  plant  consumes  these  materials  or 
assimilates  its  own  products.  Now  all  the  organs  of  plants, 
whatever  be  their  form,  their  nature,  or  their  destination,  have 
for  a  base  the  same  immediate  principle,  cellulose ;  but  starch, 


DISPERSION    AND    GERMINATION   OP   SEEDS.  229 

dextrine  and  sugar  have  precisely  the  same  chemical  composition 
as  cellulose.  Thus  it  is,  that  the  store  of  nutritive,  though 
unassimilated  and  insoluble  starch,  with  which  the  seed  is  so 
copiously  provided,  is  by  the  forces  of  nature  rendered  soluble, 
and  converted  int^-dextrine,  sugar,  and  finally  cellulose,  the 
substance  which  constitutes  the  very  basis  of  all  the  vegetable 
tissues,  it  becomes  the  source  from  whence  the  embryo  derives 
the  materials  of  its  nutrition  and  increase. 

These  chemical  changes  in  the  substance  of  the  seed  soon 
awaken  its  dormant  vitality.  We  see  the  radicle  of  the  embryo 
descend  through  its  swollen  and  ruptured  integuments  into  the 
earth,  whilst  at  the  same  time  the  plumule  rises  into  the  atmo- 
sphere, carrying  up  with  it  the  young  cotyledons,  which  soon 
unfold  in  the  form  of  two  white  and  opposite  leaves  above  the 
earth's  surface.  Exposed  to  the  action  of  light  we  see  them 
gradually  change  their  color,  chlorophyl  being  deposited  in 
their  superficial  cells.  The  cotyledons  appear  to  be  only 
indifferently  adapted  to  the  aerial  medium  into  which  they  are 
elevated,  and  hence,  as  we  have  seen,  they  sometimes  continue 
below  the  ground  without  any  detriment  to  the  growth  of  the 
young  embryo.  When,  however,  the  gemmule  or  bud  at  the 
summit  of  the  plumule  elongates  and  the  true  and  permanent 
leaves  of  the  plants  appear,  they  perform  the  functions  of 
aerial  leaves  in  a  much  more  perfect  manner ;  at  the  same 
time,  from  the  other  extremity  of  the  axophyte,  additional 
roots  are  developed,  and  the  organs  at  both  extremities  are 
beautifully  adapted  to  their  respective  media.  Germination 
is  now  completed,  the  cotyledons  and  other  appendages  of 
the  embryo  decay  and  disappear,  having  performed  their 
respective  functions,  and  the  young  plantule,  rejoicing  in  all 


COMPOUND  ORGANS  OP  PLANTS. 

the  freshness  and  beauty  of  vegetable  youth,  developes  into 
the  earth  and  atmosphere  and  depends  for  its  future  supplies 
of  food  on  its  leaves  and  roots,  running  through  precisely 
the  same  phases  of  vegetation  as  its  predecessors. 


THE    END. 


